Performance mixed with fuel economy!?

I outlined a performance+gas mileage 318 build a while ago I may build some day...

Get the compression up in the mid-to-high 9s. Run Magnum heads or something with closed chambers, iirc KB flat-tops in a 318 with Eddy heads puts compression around 9.7:1 and you'd have good quench. The key there is you can then run a leaner cruise mixture before getting into misfires and hesitation. And with that compression and good quench you might be able to run regular gas especially if the heads are aluminum.

Put a good dual-plane on top but not an Air-Gap those are too inconsistent with temperature and usually run too cold for efficient part-throttle fuel vaporization. And either an Edelbrock or Street Demon carb in the 600cfm-range.

1-5/8" headers into 2-1/4" or 2-1/2" duals... tuned exhaust helps part-throttle efficiency and MPG.

Run a good MSD-type ignition system to ensure as complete a burn as possible.

Lastly hook it up to a manual transmission of some sort, our old-school autos suck a lot of power. Whether OD or not try to set it up with a top gear final drive ratio no higher (numerically) than 3.23.

With that complete setup in an A-body I'd see 20 MPG would be a piece of cake. I had an '88 M-body with a roller-cam 318; after converting to 4-bbl, duals, ditching lean-burn computer and adding MSD box, and swapping to 3.08 gears in an 8-1/4" rear end I was still averaging around 20 MPG and that car weighed at least 3800 lbs. Acceleration was no longer a "floor it and pray you don't get rear-ended by a semi while merging onto the freeway" affair, about enough to keep up with modern traffic lmao.