This is an interesting thread. I have 273 heads on a 318 Duster. Has identical mods as yours and I ran a 15.9 (at 5000ft) with it. My next project is a 318 Volare that I'm going to put 273 heads on. I'm going to use the 2 barrel single plane intake that goes with the 273 heads and see what kind of mileage I can sqeeze out of it. I just like those 273 heads! They are lighter too and they don't seem to crack or warp like the 302's