I used ported stock 243 heads with stock valves on my LS-6 and made 425 / 400 at the wheels with just a cam/injectors and a stock intake (of course tune and headers). Would expect the ported LS-3 heads to perform even better than that. Not a bad option considering the price of new heads vs a port job. Prob about 1/3 the price. Maybe only scarifying an extra 25-30 RWHP but manny would know better from experience.