I have a 4.8L LS1/4L60E in my '51 Pontiac "Tin Woody" wagon, and an LT1/4L60E in my '48 Pontiac convert. Prior to the LT1, the '48 had a 472 Caddy/Turbo400, and prior to that, a 230 Chev six and Powerglide. I can from experience tell you that anything you can think of will fit in your old Poncho's engine bay! Check out my photo album for pics of each, and what we do with 'em.
The small block Chev is an easy swap. Super simple to make engine mounts using the old fashioned flathead style rubber insulators from Speedway (or any other vender). The stock radiators will keep anything cool if they're in OK shape, and you don't even need to think about changing the rear end, unless you want posi. Use an overdrive trans like the 4L60 or a 200R4, and you'll get good economy, and it'll be quick too.
I did swap the rear end in the convert, used the donormobile Caddy Fleetwood (a '94). A full size rear fits perfectly, I'm running 235/75R 15 tires, with the original skirts. A F or A body would be too narrow. OK under a Chev, as they are about 3 inches narrower than the Pontiac body/frame. I've had both...
Have fun!
Brian