New transmission at least, as the 4.8 uses a 6 speed. Steering rack is completely different, 4.8 uses Servotronic. The newer valvetronic V8 could potentially mount differently as well. I'm sure there's more.

As far as getting this into your 545, I don't understant why it was so difficult. After all, the 550 uses this engine. Could you not use the emissions components and ecu from it?