400 to 512

As it's a strong performer and a great foundation for a bracket car. The bore is the largest you can get which means less to impede flow into the cylinder. The mains are the strongest possible from Mopar. The pistons are very short and light... I think it's a good idea. But there will be issues with headers depending on what he runs for a head package and what he needs for exhaust.

One other thing about a 400 block is that it has more material around the main bearing bosses that makes it stronger. People used to laugh about my smaller 904 transmission but it had a riveted sprag unit from the factory and was a lot lighter than a 727. I was never able to break it. According to 440 souce and some other sources a 400 block is the best one to stroke.