[larrys40]

It can be a number if things from the shift tower and forks to the main drive gear ( front input shaft) to a 2nd high slider gear and main input shaft issue . If it were mine this is what I would do first . Drain trans, pull the shift tower and examine for any issues/ bent forks/ sliding detention issues, etc

If none found I would check the fit and slide of the 2nd and high slider to the main input shaft for fit . It should slide in freely. If not you have an issue .
You can remove the main shaft and sliders with the trans in the car by pulling the rear retainer and the rear bearing, then shaft and sliders up and out . Inspect for fit and any issues you. Can also check for cluster issues but I don't think that's where the problem is

If you have another good 2nd and high you may want to replace . Also check the pocket bearing at the front forth main shaft to input shaft . Replace shaft and bearing if warranted
You have the option to pull the trans if need be from there

Please make sure your flywheel housing shims are in as well
People don't know and realize the shims are required there
If you disassemble in this method don't forget to put it back correctly and with gasket and lock wire on thrh rear bearing retainer

Larry Shepard