I've been thinking about this lately too. If the 560 would bolt right up then I'd consider that. If you had to to serious customization then I was considering going to the later GM moderate duty trans. It's an excellent 4 speed overdrive trans and the hot rodders have all kinds of external boxes for fully programming the shift points. Who knows if it would fit in the tunnel though.

What I'd prefer is a nice five speed. I was wondering if the five speed out of a five series BMW would fit....