318 MAX fuel economy builds?

I would try and find the manifold used in this article not the airgap:

Mopar Performance Parts - '71 Satellite 340 Small-Block Bolt-Ons - Hot Rod Magazine

Only draw back was it was a Whopping 17 HP down @ 5600 RPM.......

And that is the tradeoff. But those that are looking for fuel efficiency aren't turning their v8 engines to 5k rpms.