I would recomend the 302 or 351 windsor ford V8. They are quite narrow for an 8, easy to rebuild and if you can get a late Mustang 5.0, they come with forged pistons and lots of other handy design features.

Chevy 350's are easy to get hold of and speed parts are plentiful and very cheap.

Either can make 500hp with minimal work.

For a manual box on the fords you can use a mustang T5 bell housing, cossie T5, mustang pressure plate and range rover centerplate.

Rover V8's are cheap to buy, expensive to modify. All alloy construction makes them about as heavy as a pinto though.

If you want something 'unusual' I've got a 383 chrysler in need of rebuild and a 454 chevy in the works.