'65 c10,
I would guess that the transmission is fine. The difference between a new transmission and a rebuilt transmission, if done properly, is very little.
The adjustment of the conversion kit, and condition of the little nylon bushing (dan-pa) would be the prime suspect.
If you remove the control arms, put the transmission shift levers in neutral, then work backwards to the conversion kit arm, you will triumph.
As you move the tranny shift arm, you should feel crisp detents. With the tranny in neutral, the conversion kit shifter handle MUST move left and right through its gate smoothly..no hangups allowed.
I will bet after you get it dialed in, it will work beautifully...
Stuart
