Merc 4.5 Problem

[Mercury 4.5]

I just installed a new impeller in my 1978 or 1979 (not sure) Mercury 4.5 hp outboard and can not seem to get the bottom end back on with gears shifting properly. I thought I had it but when tighting the bottom housing bolts back up I heard a snap from somewhere inside. Now when I insert the shift rod into the lower end it spins freely and doesn't seem to grab anything. I am now unable to shift motor out of reverse by turing the shift rod. I am thinking I must have done some damage in the lower gear case but I have no idea how to access this area to inspect. Not sure if anyone can help me out or if I am out of luck but any ideas would be great!

Thanks.