That A4 is using the 01E 6-speed manual which can be fairly easily bulletproofed, and with work can take 1000hp, although yes clutches get to be a challenge at that level. However, clutches in the 500-600hp range are easily available, derived from the RS4 and related things. The B7 RS4 clutch should take the V10s 450hp easily enough.
The D3 uses the 6HP series of transmissions which are bigger and stronger, but the 6HP26 fitted to the larger engines has a completely different architecture to the 5HP in the D2 - the engines are moved back in the D3 bay by 4-5" or so to improve weight distribution, hence they can accommodate an extra cylinder. To put it in the D2 would need the 01E manual again, pushed back as far as mounts and shafts can accommodate, but because the D2 is based on the 5HP the engine can't go back anything like as far as it can in the D3 because the bulkhead and HVAC are in the way, so the room needs to be found in the front.
With that much engine in front of the axle it would be an autobahn cruiser. You'd have to apply in writing to change direction
