Some folks don't put much stock in the 400 because of the siamesed cylinder walls, claim they cause heating problems, and the external balance bothers some; the heads are also different, have some extra steam venting holes in them that the 350 heads don't, and it has been said that the 400 heads are prone to cracking. That said, I never had any problems with the 400s I had in my trucks, and I worked them pretty hard; in my experience they are good engines, and given the choice, I would take a 400 over a 350; appropriately massaged, they will make excellent power and be dependable doing it.