IMM Iron heads on that Mag 318

I think they would work fine. Intake ports are only 180cc about 10 or so higher than a stock magnum head. You have 3.23 gears, but you also listed a 2.74 first gear. With a good converter and proper cam choice it should be a strong runner. Another thing that alot of people overlook is getting a carb with the correct boosters. annular boosters, or step cut dog legs, or any efficient booster "can" help with too big of ports, or cam.