Depends on several things. Unfilled, without a stud girdle, I'd say in the 600 HP area. I have seen several RB blocks fail in race applications in heavy cars at 700 HP. A stud girdle helps a lot, as does filling the block to the freeze plugs. I have several friends running 750 HP with girdles in fully prepped blocks.

Mega blocks are the way to go unless it's a matching numbers thing.