cannot get Farmall M 5th gear to stay engaged--fixed twice!

My Farmall M has trouble staying in 5th gear. I've replaced two 4-5 shifter forks so far, and no luck.

A little history---the fifth gear was working fine until about 2 years ago. Then one day the fork broke. I got a "new" (i.e. used one). And when I got it in and checked how it was shifting through the belt pulley cover, I noticed it didn't fully engage the transmission drive shaft gear when shifted into 5th. And sure enough, when tested it wouldn't stay in fifth. Looking at the fork, I think it was already bent downwards and not pushing the sliding gear far enough ahead.

So I get another "used" shifter fork. This one moved the gear and held somewhat better than the first one. But still having the same problem of slipping out of road gear.

I want to mention that both the transmission drive shaft and 4-sliding gear are in good shape. I had replaced them both a few years prior to the first fix. Also I don't do much 4-5 shifting. Transmission shaft pilot bearing is solid. As are the bearings in the transmission drive shaft cage.

When I put the second shifter fork in, I made sure to weld the end of the gearshift. Also checked the "keyways" in the shifter forks, for excessive wear. Checked the shifter balls and springs also.

A lot of these M's have had pilot bearing failure. And when that happens, the front of the transmission shaft will wobble, and bend the 4-5 shifter fork. I am wondering if almost ANY used shifter fork you might get won't have the same problem.

Any advice?

The splines on 4th 5th slider and the splines on the input shaft are probably worn. Looking good does not make good. Even a tiny angle from prior wear will push them apart. Alignment from bad input shaft bearings, or pilot bearing can also cause the alignment issue. I have under cut the splines on transmissions with a dremel tool and a diamond burr to make them "lock together. Very little material is removed (.002 from both female and male splines). Jim