A Smallblock Chevy V8 would be the easiest route. You will need to V8 engine mounts (redily avaialable from the resto parts houses) and a V8 bellhousing '55-'59-'63-'72 (salvage yard/swap meet part). A good vehicle to look for if you hit the salvage yards will be the LCF trucks, '55-'59 they were equipped with V8s from the factroy, and will have the bellhousing you will need. This is all assuming you want to keep it somewhat oeriginal and use a manual transmission. If you want an automatic, you'll want to use an aftermarket style crossmember style mounts, one for the engine and another for the rear of the trans.
just a side note, don't sell the 235 short. It migh only need a good rebuild. I have a 235 in mine, it'll run 70+ mph all day long and has plenty of power.
