yes, just make sure that both your 3.0 and the donor 3.0 both have the same crankshaft seal design. will be either a one piece or a two piece.
if all you need is a ring gear try a napa store.
I have had a problem with my flywheel continously coming loose. There is a guy who lives a few hours from me who says he has a large number of mercruiser flywheels. I was just wondering if it was worth my time to drive over there to see if any of his flywheels matched up with mine.