Basic Step by Step Procedure
Porsche 944 IRS Conversion
I have read that it is possible to fit the 944 spring plates and even torsion bars with some modification but I can't confirm if this is true, or what is involved. One last consideration to keep in mind is that performing the conversion as outlined below (not modifying the track) will result in a measurement of 58 inches from hub face to hub face, depending on what wheels you intend to fit this may be too wide (it is about 60 inches from inner wing to inner wing) , all I can recommend is measure, measure and then measure again before committing to that fatal cut, or rash decision. Please read also the section outlining narrowing the rear suspension, as a lot of this work will be intertwined with this conversion, however you may want to fit the suspension before narrowing it so that measurements can be taken. (Please note - This section is current being revised as I am rebuilding the suspension. It basically invloves cutting off the inner pivots and rewelding them back on further inwards, clearancing the rear frame and making spacers to fit between the spring plates and the A-arms - I am also working on a full custom rear setup).
Clean all nuts and bolts with a wire brush to remove any rust and dirt from the threads, then douse them in penetrating oil and leave at least overnight, I had the IRS set-up laying around for ages before I got around to fitting it, so I kept doing this every time I had the penetrating oil out for another job, it definitely made life easier, and there were no snapped bolts either.
Remove the rear suspension set-up from the car (as detailed in 'Rear Axle Removal'). This is not absolutely necessary, but does make the job far easier, if you have a swing axle car as I did, then you can either source an IRS setup, or obtain some of the weld on brackets that are available for swing axle to IRS conversions, if you are going to change the suspension then you can leave this until the IRS unit is finished and just swap them over.
Unbolt the trailing arms from the spring plates
Unbolt the trailing arms from the Pivot bushing on the torsion bar tube, this is a 16mm (I think?) Allen bolt, it will require a bit of a shock to get it going as the housing is hammered into a slot in it to prevent it from undoing, the best thing I found to use was to get the correct size adaptor so that you can use a knuckle bar to get some extra leverage. The bolt thread is about 15mm long, and you may have to turn and pull it quite hard to get it out after this. Make a note of where the washers go (towards the outside of the car), and keep these as you will need to reuse them when reassembling the suspension. The trailing arm can now be removed.
Next you will have to remove the spring plates, these differ depending one whether you have an IRS or swing axle setup, they are both held on in the same manner. Undo the four bolts holding on the retaining plates, and prise off the plate, it may be quite stiff, this is due to the suspension bushing which sits inside it, take care only to prise off the retaining plate and not the spring plate as well. The retaining plate for the IRS setup has a hole in the centre, through which the spring plate 'axle' passes, the retaining plate on the swing axle setup completely covers the spring plate, and has no hole in the centre.
Now you need to remove the spring plate, it is important to remember that you will need to mark its un-sprung position, this will give you a guide to use when reassembling the suspension. what you need to do is to pull out the spring plate just far enough so that it is no longer resting on the housing, I found the easiest way to do this was to lay the suspension on the floor and give it a gentle kick, but this was only after carefully trying to pry it free using a pinch bar with no success. Once it is in this position take a hacksaw and cut a shallow groove on the housing to show which position the spring plate should be replaced in, don't be tempted to just scribe a line as after you have painted it (which I'm sure you will take the time to do having removed it from the car you will not be able to see the scribe mark. After you have done this you can fully remove the spring plate, it is far better if you can remove the spring plate and leave the torsion bar in place, this is because there are a different number of splines on the inner end of the torsion bar to the outside end, and this will allow you to mark its position, but don't worry if this is not possible as you can use the mark you have made to find the correct position.
Okay, now the suspension is completely stripped. but before you can reassemble it all with the Porsche trailing arms you are going to have to do some machining work or get busy with the hacksaw, drill and file. But before this the first thing to do is to fit the VW pivot bushes into the Porsche arms (commonly known as 'A-arm bushes'). the hardest part is removing the old Porsche items, however I found a real easy way to do this (see pictures below). What you need to do is to remove the steel bushes from the centre and then remove the rubber inserts but leave the outer casing intact . To do this firstly prise out the steel centre of the bushing, it should not be too hard to do this, a lever bar or strong screwdriver should do okay. Now trim off the rubber flap from the end of the bushing with a sharp knife and pour lighter fluid or some other suitable fluid onto it and set light to it (make sure you do this away from anything flammable, and have a fire extinguisher at hand just in case). Provided you haven't gone to mad with the fuel it shouldn't burn for too long, when it has gone out use something like a 1/2 inch socket extension to prise the rubber centre from it housing, this will leave the steel outer part in place in the trailing arm, this part is exactly the right size to take the VW bush. You will have to repeat this a further three times as the bushes are in two parts.
Press the two bush halves in place, I used urethane bushes as they are stiffer, longer lasting and help improve the handling, with the urethane bushes you usually get a steel centre, this presses into the bush halves after they are fitted.
Right, now comes the tricky part. What you have to do is to make a new spring plate that matches up to the Porsche trailing arm, whilst this sounds like quite a huge thing to have to do it's not that bad. The stock IRS spring plate is not suitable as the bolt holes are incorrect and it consists of two plates side by side that are too close together to fit. The swing axle spring plates will actually fit on to the Porsche arms but are way too long, so here's the trick. cut down the swing axle spring plates, cut the correct profile in the end, and machine some more slots in the same layout as before (easy heh?). This is one reason it is possibly easier to start with a swing axle setup as you will not have to buy the spring plates, but can modify the ones you already have. I chose to use some adjustable spring plates. The IRS plates are correct in one aspect, the distance from the centre of the torsion bar to the furthest set of holes is the correct distance for the Porsche arms, what I did was to cut down the spring plates using this measurement, I then used the piece that I had cut off of the end as a template to mark out the new hole positions and end profile of the new shorter plate, I then machined the plate to these dimensions, I also increased the length of the slots slightly to give more toe adjustment, just in case I had made a slight mistake in my calculations. (You can fit the spring plates up with washers so that after you have set up the toe in you can weld the washers to the spring plates to prevent any further movement.) Okay now you have your new spring plates. 
If you started off with an IRS set up you will notice that the spring plates are different in design, and they will not fit the torsion bars as the bars protrude too far from the torsion tubes, what you will need is a set of swing axle torsion bars these are the correct length, if you are buying these I would recommend buying the uprated ones, especially if you intend to fit a Type 4 unit and / or Porsche 5-speed as the stock ones will not be strong enough to support the extra weight, and it will feel like you are piloting a small ocean going vessel. Otherwise this is yet another good reason to start off with the swing axle set up in the first place. The picture on the left shows the stock torsion bar on the right.Fit the torsion bars first, then the spring plates using the mark you cut earlier as a guide to set the suspension height, don't forget to fit the bushes to the spring plate before fitting it, again urethane is definitely a better choice. If you intend lowering the car, one outer spline very roughly equates to a 2 inch drop in height but this will differ greatly depending on the weight of the engine and transmission and the state of the torsion bars, other heights can be dialed in by moving the inner and outer splines by different amounts (remember they have a different number of splines, when you are happy with the height fit the spring plate covers and tighten them down (again another part that differs between IRS and swing axle cars).
All that is now left to do is to fit the trailing arm, fit the pivot end first, the arm should sit to the outside of the spring plate, fit the pivot bolt, remembering the washers you removed from earlier and tighten it up, now fit the three bolts to fix the arm to the spring plate, the bolts removed from the IRS spring plates are the correct size. Make sure that there are three large thick washers fitted to the outside of the trailing arm, they should have come with the arms (in fact the are most likely to be corroded to them) these prevent stress on the aluminium casting which could lead to breakage. An other point to be aware of is that you may need to fit a modified bolt to the front fixing, this is because of the clearance between the bolt head at the rear of the spring plate and the housing. It may be better to clearance the chassis instead to allow the spring plate to go through it's full travel without the bolt fouling.
That's it you have now installed the 944 suspension and brakes to your VW, there are still some further considerations; the brake master cylinder will have to be changed as it will not have enough flow volume to push out the caliper pistons, even if it does appear to work the pedal will travel too far, and once those front brakes are fitted it will probably not work at all. the handbrake cables will need to be modified to fit and you'll probably want to change the pads and possibly discs. All this is covered in a separate article. For now marvel at the thought that your car has super-car under-socks.
