Cool that you have an old Doug Nash case. They were one of the first performance builders of toploaders.
I think that we are onto something now.
From what I see of the 3/4 shift rail in your photos, you have the early type rails that I show here (same pic from above):
The early type uses 2 hatchet type pins, 3 dome type pins (not blunts), and an extra long interlock pin.
A closeup of the 3/4 shift rail and pins of the hatchet set:
A similar closeup of a 70' set which uses 5 of the same blunt pins:
Notice that the later shift rail does not have the deep neutral groove. I found a 68/69 set to be similar.
I compared the early style to some other year parts and discovered the hatchets are approximately .030" shorter than the blunt/domed when compared to the 68/69 or the 70 style.
Due to the .030 difference in the pins, I believe that if you get the correct parts (at least the 2 hatchets and the appropriate length interlock pin) you will be good to go.
IF the interlock pin you have is the long one, you might try removing it and then install all the other parts and see if it shifts. If so, my thoughts are that maybe a shorter interlock pin "might" get you by. Still the proper and complete early style would be more desirable and will last longer due to a larger contact patch.
Since you can measure what you have, here are some pics with measurements of the early style:
Notch in 3/4 shift rail:
IIRC, The notch in the later styles were off by only a few thousands.
Interlock pins--early on left, 70' on right:
As far as the nylon bushings, there is nothing wrong with using them and they may even be preferred. The idea behind either the copper based ones or the nylon ones is that they can absorb metal particles which helps prevent those particles from interfering with the roller bearings or other parts of the toploader.