I think you are on the right track with the quench and bumping the compression up. Is that .041 quench with zero deck pistons? The 390 carb should work too. Is the intake a long and narrow runner type? That would optimise lower rpm torque and increase mpg too. Those small valves will be dandy for higher port velocity too as long as they aren't the choke point. Polish the combustion chamber and maybe coat the piston tops too. I think Speedway has some ceramic spooge for that. You can look into some of the tricks that the emissions cars have been using for economy like electric fans and running a tad hot plus the cam grinds too.