There are TWO numbers being quoted, thus the confusion. GROSS hp on a 318 2bbl was 230. Check a 1971 318 2bbl horsepower rating (before they went to net). So yes, the 4bbl, heads (good head choice), and cam with dual exhaust should put you around 310-320 IF it is tuned, timed, and jetted properly. The answer to your question is headers will not benefit your motor at your driving habits, under 3500 rpm's. The manifolds will help you on the low end torque vs the header.