920 heads - What Cam?

I used a comp XE250H in a similar setup - mid 70s 318 with ported closed chamber truck heads (very similar to the 920 castings) edelbrock performer manifold with 625cfm carter AFB, headers and 2.92 gears. Good power bump from stock but nothing crazy, it was a pretty good setup for a low buck daily driver.
You could easily go 1 size bigger on the cam (XE256H) seeing as you have a early 318 it should have taller pistons than the later stuff so you should have a little more compression, the 3.23 gears will also help. Comp 901-16 spring will do you for either cam.
You don't have to use a comp cam but something with similar specs will work well.