Jim is right in his description of a 391 FT engine but they were fitted to large trucks, not pickups and such. I doubt a 391FT would have been mated to an auto trannie in a large truck but you never know. The oversized crank snout and large distributer hole are the dead giveaways. As long as the blocks have 3 bolt engine mounts, the early FEs had 2 bolt, I would go with the one that has the closest bore and crank sizes to std to allow for safe cleanup. You are probably going to throw the
oil pan and manifold away anyway. Either block is good for 375-400 HP with little effort.