Having solved the leak issue on my OD transmission (the apparent OD leak turned out to be a leak from the selector rods on the shift tower) I decided to practice on a spare transmission before tearing into the second issue; slipping out of gear. I did notice that this problem was probably addressed by the precious owner since there was a “distance piece” on the second gear detent that I don't think is correct.
Here's the background on the spare non OD transmission. I bought it with a wrecked parts car fifty years ago and at some point replaced the top cover with a piece of plywood