I think his point is, if you're going to drop a non-Mopar engine in a Mopar, why limit yourself? In fact, the small block Chevy would be even cheaper and easier. If you're making such a radical change, who cares if it's still a six?
FYI, some people would consider what you suggest heresy. I'm not personally offended, but I still wouldn't DREAM of dropping a non-Mopar engine in my Swinger. If I wanted an M3 engine, I'd buy an M3.