I think a 318 or 360 with true 9.0 - 9.4 compression, a stock 360 cam with a nice dual plane intake, Holley 600 vacuum secondary, and dual exhaust would be a nice street economy engine that you could take down the track occasionally and see if you could get 20 MPG with it on the highway...
This is the basic thing that was done many moons ago which is most effective when you start with the earlier 318’s from the 60’s with there higher compression starting points. Or any older engine for that matter.
The best tricks to make that stock OEM using equipped car were to kill the heads rod that edge in compression, 3 angle valve job (or a race valve job in todays standards.) and if you had enough money it was a port match and porting to gasket size on the heads and intake. Bigger valves if the wallet was thick enough. That was a lot of money in the 70’s. Aluminum heads were not available and the only upgrade for the small block back then was the W2 head. Very pricey back then.
Upgrading the cam was always part of that performance combination along with headers. This was always proven to be a very good street combination.
It’s much harder to get good results from low compression engines from the smog plus era. I’ve measured late ‘70’s 318 as low as 7.5-1. That’s Yak piss compression. If you could get to at least 8.5-1 or better, a small cam would work very well and still use 87 Octane.
My old Magnum’s ’79 360 was a fair performer with its stock long block and use of factory iron intake. The newer ‘00 - 5.9 is much more powerful. I do t know if I’d would get 20 mpg’s with it because I went from 2.76’s to 3.55’s & get about 17 now. It’s been a while. I also went from a 904 to a 727.
