Re: Oil Pan
I'd be concerned about the rear main seal type of both engines, 1 or 2 piece. I'm not sure on the years, but the the GM 4cyl marine engines came with a 2 piece and 1 piece rear main seal blocks. I don't know for sure, but I'm guessing the 96 3.0 has a 1 piece rear main seal and that the pans are different.
BTW, have you checked your local GM dealership for an oil pan for a 68 Chevy II with a 153CI 4cyl? It should be the same thing as your 84 120HP Mercruiser, oil pan at least...
As Bruce said, slap a couple coats of paint on the pan before installing it.
Oh yeah, splurge and get the 1 piece oil pan gasket, much better than the 4 piece set.