Take the shifter lever off of the tranny side cover and see if the rectangle on the shift arm is worned so it is not square on the ends that the shift lever fit onto If so that may be the problem or part of the problem If not look remove the side cover and look to see if it is a early cover with rooster combs and brass shifting forks or the later one with the GM style cover with steel shifting forks. If it is the early one make sure the check balls, one on each side of the srping are in place, also look at the half moons that the check balls ride in. If those check out good the last thing that can cause what your having is the slider and the engagement rings on third gear being worn,not uncommon Or maybe the snap ring that holds the synchro hub on the main shaft (output shaft) is loose or missing Lots of gremlins out there, especially on used race parts Good luck, let us know what you find. BTW,I had a set of gears that I had to hold the shifter in third gear or it would pop out I ended up having it redone(new engagement ring on it, and put a new slider in the box and that fixed that 3-4 gear set