Consider a 318. Plentiful in the yards, cheap, powerful, they fit pretty well from what I gather. These trucks came with a smaller form factor Torqueflite. There are a few pics of those swaps on here, they mostly look very cleanly done and they don't look out of place in the engine bay. I'm not a V8 guy but if I did a swap, it would be a 318. Also that would keep it technically all Mopar, which has to count for some style points.

Check out Fordubishi's 302 swap thread, too. Very nicely done.

SBF and SBM seem to tuck right in with a little work.