From what I've been able to understand prior to '68 everything was forged. '68-up 318s and 273s could get cast. All 360s were cast, and '72-3 340s are cast. I've got a '71 built '72 cast 340 now. It seems as though sometime in the later '70s forged cranks for 318s went away.
'72 340s are also external balanced. They have 1.88 intake 915s like early 360s while '73s have 587s.
In '67 the 318 was offered in Bs, Cs, and trucks. In '68 it replaced 273-4s for As. The forged cranks seems to be like the rears and transmissions- you'll see all sorts of applications it exists in and there's not a lot of continuity for the "why". 1 ton farm trucks could have cast cranks because the buyer was cheap, and an A body could have a forged crank- there wasn't much pattern once you left standard. Many later A bodies with 8-3/4s were 6 cylinder cars while it's not at all uncommon to see 7-1/4s under 318 cars as a bit of an example.
One thing you won't want is you won't want to mix a basically stock cam, with small chamber heads, with high compression pistons- the cam you run will be relevant to how well you can run pump gas as will the other factors.