68 truck 318 cam. How big can i go?

Adding rpm is basically like adding cubes to your engine, everyone likes to talks that engine A is bigger than engine B (eg 318 vs 360) but that's only in a static condition but running your engine size dynamic.
eg (360 x 5500 rpm) ÷ 2 = 990,000 cubic inches
(318 × 6225 rpm) ÷ 2 = 990,000 cubic inches
( you can drop the (÷ 2) since no one cares about the exact size so if you want to ballpark what a 318 would have to turn compared to a similar built 360 just (360 x rpm) ÷ 318 = new rpm ) But at those rpm those to engines can both pull the same amount of fuel and air per minute having the capability to make same power.

But I think your talking about is that a low cr engine has a few more cc over a high cr engine at the same VE yes but the small gain wouldn't make up the power lost by inefficiency of the low cr for the gain in a few more cc.