B5 2.5 V6 TDI in to an 8L?

No, the a4 engine is mounted the wrong way round compared to the a3.

Second that ! Not enough room to fit the engine but if you decide to cut all the inner wings and fabricate new suspenssion mounts you might be able to squize it in ! Just like the boys from Dahlback racing or the VW TDI Racing Team(mind you, they used the 2.2 inline-5 cylinder turbo 20V engine as fitted to the s2/rs2 engine)... It will need fabricating a new firewall, transmission tunnel, shift linkage and new geabox mounts... If you have the time and skill+ tools, everything is possible.