Are you dealing with a front intake or rear intake OPS? The front intake will require a new crankshaft with the porting changed for reverse rotation. Although I seem to recall that on the K&B 3.5 with the removable crankshaft housing, it was possible to rotate the front housing 90 degrees to get reverse rotation. I think that was done for the outboard version. Does you OPS have a removable front housing? On the rear intake, the rotor will have to be timed for reverse rotation or you might be able to rotate the rotor housing to affect the timing. I can't tell you if either the crankshaft or the rotor are available for reverse rotation. I'll take a guess and say that the parts will have to be custom made.