The shift shaft may still be connected. That's usually the first thing you disconnect when removing a lower unit. Mercs typically have a splined shift shaft that simply slips off on it's own however, many of the smaller models back then were actually made by Yamaha. Don't force it. Have a look at the shift shaft and see if it moves up and down to shift gears or if it rotates. If it rotates it's splined. If it moves up and down it needs to be disconnected somewhere close to the lower unit. Some of the old Mercs have a lower unit that's slung back more than normal at the bottom. I'm not sure off the top of my head if your's is that old but I think it is. In that case it wouldn't drop straight down necessarily but pull it a slight to the rear as you lower it and you'll notice it seems to hang or bind on the rear stud. It goes on the same way, a bit difficult and may take a few extra attempts.