Small block Chevy is just about as compact as you can get in a V-8, but for talking purposes, a Caddy is not much bigger. There is apparently enough room in that hole, though, for a big block, if you relocate the steering box. Getting parts for a Chevy swap of either size is quite easy; for other make engines, not so easy. My own personal thoughts for this particular type car (earlyChevy) run to a stovebolt, because I just think the sound of a nice set of six cylinder pipes fits my memory and idea of this car. I'd try to find a good 235 or 261, or a later 250 or 292, and sweeten it up. With the early engine, it should be a direct bolt in; the later one will likely need some mount work, but that would be needed for swap to a V-8, so no harm.

Some food for thought, eh?