Stock 72 318 power gains

You're not tapping much airflow with that camshaft and those heads don't flow that much Peak anyways without opening the valve beyond the usable range of lift. The heads working better would in turn make the camshaft work better. It's not the cubic inch that limits you it's the heads. And giant heads would suck like Edelbrock or something, total Overkill. Those stock 675 heads can be made to flow 200 CFM by .400 lift easy. The 318 head porting for the Average Joe have flow numbers on the first page for the casting heads you have so whatever your cam lift is...you can look at the lift corresponding to the flow number and that's all you're getting multiplied x 2 for power.
Just putting it out there.