273 or 225?

Having hot rodded both engines, I'd do the 273 swap in a heartbeat since you have a 273 donor car where you can get all the special V8 parts. The only problem, and not really a problem, with a 65 273 is finding an intake manifold and there are work arounds for that as well. A 65 273 should have a solid lifter valvetrain, a forged crankshaft and good heads. My 64 Barracuda 273 had the same power to weight as a stock 68 Roadrunner with 383. Try that with a slant six. That being said I love slant sixes. You can't lose either way you go.