That does not cause jumping out of gear. The spring in the trans cover could be bad or slack between the input shaft & mainshaft. The teeth on the syncro unit can be bad. You are correct, there is not much to go wrong in the selector box. You can build up the parts with weld & sand down the mating surface of the cover to snug it up.
Thanks for the info. I do have the back up trans that I plan on installing as soon as I get the engine for the fly wheel. I will clean it up, replace the clutch at the same time.
