NP made two different styles of the side covers that went onto the NP833 trannys put in Mopar 4 speed cars, the early ones where used from 1964 to 1971 or so. The early ones used the rooster comb type selectors, the later ones used the scissors type side covers like GM used with the better steel shifting forks
On your deal I would disconnect all the shifter rods from the shifting arms and make sure the tranny is in neutral and then see if you cam manually shifted the 1st and second shifter arms on the side case, same thing on the 3rd and 4th arm
if you can't move them then the tranny has issues inside