Untouched or little work 318 heads would have a hard time to break 300hp with low CR 9:1 and less and street cam 270 or less. It would take a lot of CR and duration to get much above. 340/360, magnum or ported 318 are the way to go. 340/360 are a good swap with high CR piston gets you a streetable 9:1 CR with enough flow to make decent power with a streetable 270 ish cam.