Peanut Gallery Survey for my /6 Build ...

Typically HP and mpg are not mutually exclusive. When you up the efficiency of the motor, you will get better mpg if you can keep your foot out of it. I'd mill the head about .060 if it has not been milled. Get a cam in the 268 degree duration and about .450 lift. Add a double roller timing chain, 340 valve springs, a windage tray, a high pressure oil pump spring from a big block, an electronic ignition from a 73-74 /6, a Super Six intake and carb, and the exhaust from a Feather Duster. Enjoy.