I was getting 22mpg with my closed chamber 273 heads on a 318 and running a RV cam. It will be real interesting to see what you end up with. My engine was non quench with a 500 cfm carter carb and 2.92 gears, also lock up trany. I run at 5000 ft. Assuming the correct jetting I wonder what the effect of altitude is on gas mileage? Looking forward to your results