with way too much work, we have had 516 heads flow in the mid 280's/190's with the 2.08/1.60 valve sizes. In general, the port flow is terrible in stock form vs the 906/452, the castings are thin in the intake ports, the short side on the exhaust is non-existent, the exhaust ports are thin on the "hump" area on the side of the port walls. After lots of work, yes, we made 580+ HP on a 383/carter carb, ran a best of 10.35 at 3470 lbs, then they cracked through the exhaust ports. In general, too much effort for the results.