Slant 6 Gas Mileage

You have something dragging you down thats for sure!! Brakes, bad alignment, rear end..
I get 20 to 24 miles to the gallon easy, what ignition and plugs are you running ?
I hope the new build loosens up some, but dang!! 9 mpg park that puppy on a level
parking lot and but it in neutral and push it and see what it does and sounds like..
Carb has got to be running fat/rich to eat that much fuel or it is running under a load of some kind.