Compare the synchro teeth of 1&2 to 3&4 teeth. Probably will fine 1&2 alittle ragged.
Not much you can do, 1. If at a stand still, clutch disengaged, shift to 4th to stop rotation of cluster shaft, then try 1st, 2. Replace trans oil with Redline MTL fluid, 3. Remove trans, replace synchro rings, bearings, gaskets, shim it for proper clearances, re-install transmission.
Muncies and T-10 were well known to leak oil.