RWD converting a quattro wouldnt be all that difficult, You'd just need to swap in an RWD box and knock up some mounts for the transmission.
Remove the front driveshafts, leaving the outer CV's in the hubs to keep the bearings properly tensioned.
The Volvo M90 box from the TDI 940/960 uses an audi bolt pattern which is the same as the 5cyl engines (the volvo TDI of that era is infact a 6cyl VW unit which shares its bolt pattern with the audi 5Cyl and therefore the V6 and V8 too (not sure about the 1.8T but i suspect it might also be the same)
Add to that the fact that early 2.6 Quattro's sell on ebay for as little as £600 it would probably be quite a cheap project
I certainly wouldnt want to start cutting floors out though, makes much more sense just to buy a quattro in the first place