Well, you don't tell us what flavor of A body it is and it seems most assume it's 67 and up.

Which if that is so makes a swap easier. The early A bodies are much tighter in the engine compartment. If it's a later A body I'd go Gen III hemi, which you didn't ask about. If it's gotta be a SB or BB only, then it kinda depends on if you want a nose heavy ride or a more balanced, aluminum headed small block of however large you can build it would be my choice.

If it's gotta be a BB, then a poked and stroked 400, the B block fits easier than the RB.

If it is, indeed, an early A body, then an aluminum headed SB is the easiest to fit in there and is a more balanced package if you care to turn at speed.