4 cylinder engines are actually stronger than a slant 6. They have 5 mains for 4 cylinders. Chevy Ford and AMC 6's have 7 mains and can handle a lot more boost. A slant 6 only has 4 mains for 6 cylinders. THe blocks and cranks must be strong to last as long as they do. I wouldn't push too much boost into one though.
