I just put in a 383 small block,believe it making 350 hp. Ran it got 62 mph at 5200 rpm with stock alpha drive (please hold together) with a quicksliver three blade prop. It had before a 350 mercruiser magnum rated at 260 horse and would run at 58mph at 4900 rpm both speeds check with GPS. On the 383 not sure if the Vaccumm secondary are opening. The 383 big difference was the pull that it had got their quick to 5200 had stayed. Thinking of increasing the prop. please make suggestion, comments anything. Wanting to hit 70 mph.