Duckluv, there's a lot about this that I don't know, so please accept this as an idea / opinion. The Power Shift is is an adjustable prop .... as little or as much pitch as you want. Measure from the centerline of the crankshaft down to the transom, and if you've got enough height in your engine stand (37") so that you won't be hiding more than about 3 inches of the the prop behind the transom with every revolution, go with the bigger 74" prop.
You won't have to turn it as fast to get the same push, so it will quieter, and you'll burn less gas.
MM Pat, help me out here. Did I tell him right?
K.