The heads won't make any difference (assuming they have the same combustion chamber size as your 273 heads), but the four barrel will. But don't expect muscle car performance. We put a four barrel and dual exhaust on my Mom's 64 Barracuda (using the rare Edelbrock 4-barrel intake for the early 273 heads), with stock 904 trans and 2.94 gears, ran 16.70s. About a second quicker than stock, not as fast as a stock Commando 273 (less compression and milder cam). Since your 273 needs a rebuild anyway, I agree with the above suggestion just to use the complete 318 motor. That's 45 extra cubic inches, more horsepower and a lot more torque. Eazy peazy. . . .
