building a long distance touring car

Doing it on the cheap and easy, ether the 5.2 is a good route, fuel injected or not. The real key here is to be easy on cam timing. IMO, a stock camshaft is the key to any mileage. The second part to it would be tire size coupled with the final drive ratio & where it cruises at rpm wise.

Krooser suggested a Q-jet, which I’m not familiar with but I am with a TQ which is of course a spread bore. The small primary version is a good idea. IDK if Rochester’s have different primary versions. If you can find/rebuild a good TQ, that would be a good candidate, but a spread bore is a cool idea. Otherwise a 600 cfm carb will do ether engine well.

I did a RPM carbureted 5.9 swap with headers/2-1-2 exhaust, a 727 & 3.55’s on 245/60/15’s.
I did get the mileage you seek.