Are you just swapping in that non-H.O. as it is without going through it first? If so, that's just a total waste of time and effort. The least you could do is swap in a decent H.O. cam that will work with the cast pistons that came in the non-H.O. motors. Maybe something similar to the B-303 as far as specifications are concerned. Install a new oil pump and screen (if the existing screen isn't clogged up. Do all new gaskets all the way around before dropping the motor in. Preventative maintainance will save you tons of potential headaches.