When a gear ratio change is made on two meshing gears the pitch diameter is changed. This requires both meshing gears to be changed even though one gear may have the same tooth count.
The pitch diameter of the 23 tooth gears should be different on the countershaft gears when the tooth count of the mating gear on the main shaft is changed for a properly designed set of gears.
I think you will find the following if you measure the outside diameter of all of the 6th gears:
The 26 tooth gear from the 1985/1986 6th gear on the main shaft will have a slightly larger diameter than the 25 tooth 6th gear on the 1987/1989 mainshaft gear.
The 23 tooth gear from the 1985/1986 6th gear on the countershaft shaft will have a slightly smaller diameter than the 25 tooth 6th gear on the 1987/1989 countershaft gear.
The transmission would probably go into the cases if you used the 1085/1986 23 tooth on the countershaft and the 1987/1989 25 tooth gear on the main shaft, but would have excessive backlash.
The transmission might not go into the cases if you used the 1085/1986 26 tooth on the mainshaft and the 1987/1989 23 tooth gear on the countershaft, because the combination of the pitch diameters will push the two shafts too far apart to fit into the case bearings.
