It's kinda weird, early 273's only. Fall flat about .500 and go nowhere or down. I have seen it from more than one source. Those numbers are unported heads. Actually headers might not seal the small exhaust port on the 273 heads, although I have run 340 exhaust manifolds on the early heads. Something to check. Plus 10 cfm is good, not sure what the end result would be on his combination. He has both options available, his decision. Actually, I'd go 273 all the way, big surprise huh?