Yup, a bad/worn/missing pilot, whether bushing or bearing, will allow the inpoot shaft to "flop" around, and force the shift collar to dis-engage. Fourth gear is made onto/part of the inpoot shaft. Many a time this has been the source of the problem, as well as the bellhousing not being flat against the block, such as when some Goober mashes a wiring harness in there!

Hey, ya'll. Wanna see what was wrong with Longhorn294's(Dwain) trans? He said it was falling out of 3rd gear, and here's what I found.
Here's a pic of his 3rd gear

The gear on the right is brand new, and has sharp, pointy synchro teeth, and that lets the coupler "hook" onto the gear, and hold it in.(it's called "torque-lock)
The gear on the left is out of Dwain's trans, and the teeth are completely rounded. The corresponding teeth on the coupler are worn even more.( fergot to get a pic of that! Will add it later) When the teeth are that worn, the coupler will not hook/lock onto the gear, and eventually just "falls/jumps" out of gear.
The rest of the trans was in OK shape, but we went ahead and replaced both shift forks, all bearings, brass(synchro rings), and seals.
Still waiting to hear if he's got it installed yet.

Thanks for posting that picture Joel. It's nice to see the comparison between a new and worn synchro.

Joel,

 You'll be hearing from my lawyer. You had no permission to expose the private parts of my transmission. Jeez, is nothing sacred anymore?   It was susposed to be installed this evening. Monday the MSD ign. system is going in along with some carb readjustments then replace all six u-joints and if time permits, pulling out the seats, console and carpet and put down insulation and a new console. So much to do, so little time! We'll git-r-done. Many thanks to Joel for the tranny work.

Sorry, Dwain...once those private parts are exposed,(i.e.-removed for viewing) you have no reasonable expectation of privacy. In fact, it is my civic duty to report such exposure.

Jes doin muh job...