4bbl Carb questions

I understand the 2-barrel "Super Six" manifolds were common in later years like the Aspen and Volares, but not cheap. There are 4-barrel to 2-barrel aluminum adapters that probably fit (~$10 used), but then you may need a low carb for hood clearance. A 4-barrel aluminum manifold is better, but expensive. If you go 4-barrel, get the smallest possible, say 350 cfm. In theory, you can get better mileage with a 4-barrel than a 2-barrel, especially if small primaries ("spread-bore") and vacuum secondaries. Look at the Holley Economaster.

Longer term, consider changing to throttle-body injection. I recall some have adapted them to Slants. You probably need a custom adapter plate. Harder is the controller, like Megasquirt. The Holley Projection is easier, but 670 cfm is big for a Slant. Much Slant Six info on FABO, also www.slantsix.org.