New bb build fuel economy

Plym,
How much are you willing to spend to get good economy?
For best economy, a low duration cam will be needed. So any of the factory heads would be suitable. Early 516 heads are closed chamber, will be very efficient with quench pistons, but might be hard to find.
I have milled Chrys heads 0.120", zero problems, to gain compression. You need to also mill the intake face for port/bolt alignment. The Edel Perf [ not the RPM ] will be ideally sized & use a TQ carb on it, you can get recon TQs from Summit & probably the sponsor, Woodruff carbs. Mate bought two Comp Series TQs from Woodruff, plus jetting kit, they were very good.
High compression [ for efficiency ] & short cams with pump petrol are a detonation challenge.