318 valve sizes are the same as 273, just a slightly larger chamber. You can use 318 heads if you use both. Small valve 360 heads make a nice upgrade for a 273 and do make more power but they take a higher level of execution. Porting exhaust manifolds, centering chambers over the bore, intake and carb changes, then making the rest of the drivetrain to hold it. It also pushes the power band higher. Not a simple way to go.
