318 MAX fuel economy builds?

A stock 2 barrel teen with a 2.xx rear gear will get you 20+. Keep your cruise rpm less than 2,000 and you will get there. That's one of the reasons why all the new cars have 6 and 8 speed transmissions, double overdrive and the like, to keep the cruise rpm down. If I was going for mileage and efficiency I wouldn't want to do much with those 920's other than a nice valve job, a back cut, a little bowl blend and may be port match. You can make them better without turning them into a J head.