I've thought so far about the following engine choices:

5.3 vortec v8 engine
audi 5 cylinder
audi v10 engine
lexus v8

All of those should go in without any major issues from what I've seen, biggest issue might be the v10 length but even that I don't think would be the end of the world.

Gearbox is more of a pressing issue currently. Trying to work out what would be the best route.

Lexus - I've heard a bmw box would be reliable, cheap and easy to match up a rear diff.
the audi stuff is more complicated as I could use a quattro box converted to rwd. Annoyingly my 6 speed v8 box is still in the UK otherwise i'd have tried it in the hole