When you put the gearbox together was the fork fully seated in the cradle and did you get the pin through the shift fork? Did you check it shifted properly
If you had the lower unit off to do that job......... The upper and lower shift rod have a groove in them that must align with the holes in the brass
When I took the screw off . The outter tube did not fall out . I took a screwdriver at the top and pushed down and it would not pop out . That is when
The OP says: ......... of which typically means "Closed Cooling" system.
The closed cooling system side
It's likely they never had a manual for the boat...for the engine & drive, the service manuals would be the things to get...Mercruiser has the basic