318 0r 360

On the bright side, you won't have to change the K member since you have a 65 and all 63-66 A bodies used the same K for /6s and V-8s. You will need V-8 brackets and mounts for a 273 if you go with a 318. If you go with a 360 you will need brackets and mounts from Schumacher. As to engine choice, 350 hp is dirt simple with a 318 (Magnum heads, 750 4 barrel, 264 cam, headers) but for me the real dealbreaker was the headers for a V-8 in an early A body. About the only option you have is fenderwells and they are a royal PITA to live with on a daily basis, not to mention the price. Good luck finding the 273 exhaust manifolds. My son is getting a 63 Valiant 2 door post with a slant. You would be amazed what they will do with not a lot of work, and they are a real hoot with a 4 speed and 3.55 gears. Just my .02 worth.