Unfortunately, that little transmission has a bad habit of getting "worn out all over worse than anywhere else!" Back when these vehicles were daily drivers, lots of Chevy parts departments kept brand new complete transmissions in stock- - - -it was usually less expensive to replace than it was to rebuild them. From your description of the problem, it sounds like the 2-3 synchronizer assembly is badly worn or has some broken pieces jamming it up. That part is very scarce, and also expensive when you do find one. I'd strongly suggest installing a "small parts kit" that contains all the bearings, thrust washers, and gaskets in the transmission, plus replacing whatever "hard parts" you find worn or damaged once you open it up. Make 2 or 3 more posts, and then send me a private message- - - -I just might have a simple solution to your problem.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!