It could be something as simple as the tooth on the select lever that's jumped out of the groove on the shift shaft coming out of the transmission. Have a close look at the following pictures to get an idea what I mean:
This is the 4 speed Muncie with the levers circled in red:
One of the levers causes the shift shaft to rotate while the other one causes the shaft to pull out
of or push into
the transmission. The green arrow in this next photo shows how the cable pulls and pushed on this second lever, while the red arrow shows the resulting movement of the shaft, and the blue arrow points to the location of the tooth on the lever that engages a slot in the shaft.
Often times the little tooth on the lever gets worn to the point where it slips out of the groove on the shaft and no longer allows you to shift. A welder can easily build up the tooth again and grind it so it's like new.
Hopefully that's all that's needed.