What engine would you build for a Ramcharger?

Alright, I'd go with applying some modern engine techniques and management to either small block.
To me, improving combustion efficiency and reducing detonation sensitivity is job one for mpg, so I'd use whatever heads with the best combustion chamber, tighten up the piston to head clearances ( zero deck or go slightly out of the bore and use the head gaskets to get things right ), put at least 10:1 compression into it and get a cam ground that will make the dynamic compression compatible with junk 87 octane gas. Add a small runner dual plane intake, carb to suit and the REAL SECRET SAUCE: a J&S Safeguard knock controller. Tune for best power and mpg, add the controller and it will keep this analog combo out of detonation. Look into the unit, it's been around for a very long time.