Here's a head shootout on a 302/306 with 9.##:1 cr, xe264hr cam same as the 350/360 hp 5.9l magnum
had, air gap and headers, made least 306 hp with stock heads (similar to 318 heads) and most 396 hp with AFRs.
Shows without going crazy on cam 212/218 .512" 114 and good heads you can make good power even with a 306.
Same cam as 306, 350 hp 5.9l think he got to 360 hp with an air gap on engine masters.