I'm doing it - almost finished actually on my Sprint.
Its not as easy as it sounds mate
The rear axle of the p4 is straight - obviously - so the drive axles can go straight to the wheels from the diff.
The rear axle of the sprint or 33 is angled.
This means that you need to make adjustments for the rear suspension. What I did was cut the plates of the rear coilovers and moved them 14 cm's up. By cutting the coilovers you will get an extremely short suspension of the car, with the result of the car bouncing around instead of working with the suspension.
In addition to that you need to make brakets for the shock absorvers so they can mount , again around 14cm's further up.
There is no other way around that.
Then you must ofcourse remove the fuel tank and in my case I had to chop out the rear spare tyre floor panel to make some more space.
With that done, depending on how low you want your ride to be, you need to make additional clearance for the panhard rod of the p4. I actually choped the rear frame of the car to make clearance and strangthen it again later.
Then, you need to fabricate mounts for the center axle that gives drive to the rear diff as well as for the gearbox which has a different mounting position than the standard one of the fwd.
Finally, if you chose to go rear wheel drive and not 4x4 like I am, you will need to dissasemble the cv's from the front axles and mount the cv's only on the front hubs so that your front wheel hubs dont come apart while driving. You will also need to make covers for the gearbox to remove the inner cv's.
Then you need to either lock the electromagnetic switch on the gearbox that gives power to the rear diff or just connect it to 12V constant - I'd recommend the first option as the box is a bit fragile !
..... thats about it !