I know if I was building one, I wouldn't want the big port intake on the small port heads. Obviously, on the factory stock 318 it wasn't an issue, but I'm thinking on a performance oriented 273, it could be a spot for reversion.
I remember reading threads here with spirited discussions about port mismatch. I don't remember seeing time slips or dyno sheets though to prove or disprove the theory. I think it's more of a "doing it right" thing. Will it run ok with a mis match? Yes Will it run better with port matching? Probably. Will that difference be notable on a dyno, at the track, or the seat of your pants? May be.