Swap 115 hp lower unit onto a 90 hp?

I have a 1992 90 hp Mariner with no reverse. I bought a very similar lower unit from a guy on craigslist. He thinks it came off of a 2004 115 hp Mercury. He thinks there was something wrong with it but really wasn't sure. Hey...,It was only $100. I was hoping to swap units to see what might be wrong with the new unit or possibly using one of the units for parts. However, it looks like the only difference between the 2 is that the new driveshaft is about 5/8 of an inch longer with a flat top as opposed to a tapered top. Common sense tells me not to do this though. Should I Swap shafts? Remove reverse gear parts from new unit, hope they are good, and use them on the old unit? Or any other ideas. If you were in my situation what would you do?