When you get the service manual and I hope it is a GENUINE OMC, you will see the drive being worked on mounted in a special holding fixture. Without them it is a real PITA to work on.
I have performed this many times, for you I suggest first drain the upper gear housing. It is not necessary to pull the drive off. Tilt the drive 1/2 way up disconnect the shift wires and pull them out, there may be a spring attaching the wire to the rear of the upper gear housing you have to remove. If this boat has external steering disconnect it. Remove the 5 bolts from the top cover and the lower will come off. These bolts are 1/4” 20 and break off easy if this is a salt water boat, you may need a torch to heat the area where they go through the cover. Remove the four 3/4” hex head bolts and the whole assembly will pull out by rotating it back and forth. Have a drain pan under to catch some oil that is still in the housing. From this point on use the service manual.
There is a very good the possibility that the overheating is being caused by the pipe that supplies water to the swivel bearing where there is a rubber grommet that gets pinched due to corrosion. Refer to the service manual on the procedure. If you are lucky you will not damage the shift wire. If you do let me know and I will be able to help on what to do.