The R32 boot floor thingy is to house the huge Haldex unit, I thought that Audi would probably fabricate all b6 shells the same, as it makes manufacturing and economical sense, then leave off the bits not required.
As the Torsen is purely mechanical and self contained, it might have been simple to fit as a retro. The b5 and b6 versions dont even have the stupid torque split switch so need no in car control at all.....you would (if the shell was the same) only need the torsen centre diff, propshaft and rear axle?