Go with 396, sb 400 don't have water jackets between the cylinders. I use to race a 73 camero with a mildly massaged 396 and it was great. Theres not alot to learn on big blocks, except when you mash your fingers you yell louder. More torque and hp for not much modofications.
Besides I like saying 396, 396, 396, sorry I got carried away.