You do not understand. A 318 "all" things being the same, will run very similar to a 273 but at a lower rpm. A 273 was a high performance engine with a forged crankshaft so no matter what you do to a 318, you have to do a lot to catch up. But not much of a price difference when rebuilding an engine, pistons, cam, etc. I also include 340/360 heads, carb, intake, distributor, and exhaust. You don't have to do anything to a Commando/Charger 273, it is all there, maybe a slightly larger cam.