Dave, there were a bunch of different things OMC did along in the years that followed your motor to help strengthen the gearcase as to jumping out of gear. Finally they went to a three lug gear and dog arrangement and that took care of the jumping problems. Like Frank said the parts were expensive. Some of the Kit #s were 390970, 390972, etc. Of course the final kit took everything. Both gears, dog, bearing, etc. I remember caring around some of those kits for customers who had repeat failures and needed the final fix. Most of the worst problems came along after the engine went to 32 cu. in. In 1977. That was far more power than that gearcase was ever designed to take.
Most of the time if you have someone who can put a 2 degree ramp on the gears and shifter where they will pull together rather than push apart that will take care of the older 26 cu. in. motor. Hope that helps.
Dan in TN