225 slant 6 or 360 v8

Be aware that when stuffing a 318/340/360 into an early A body, then the engine compartment gets really tight in a hurry. So do a bunch of reading up on fitting fans and radiators, and the exhaust systems you can stuff in there with a V8, before making the plunge. It is all doable; just be mentally prepared for it to be a bit of work.