273 commando HELP!!!

I did a 273 for a 65 Valiant 4drsdn, way back in the late '70's or early '80's. I don't remember all the specs (would had to find my old notes) but it ran 12.80's-12'90's in the quarter.
I do remember it had Jahns 12.5 pistons. I did try a set of 360 heads but saw no improvement over the 273 heads. I think it was a case of what was gained in flow (bigger valves/ports) I lost in compression (open chamber vs closed chamber). Cam was a Crane with about .520 lift, dur ?, intake ?, carb Holley 750 DP, Fender well headers, dual point dist. 904 with a good (at that time) converter, RMVB, 8 3/4 (don't remember ratio) rear.