Woops, made a mistake on the primary coil wire routing.
I had misplaced the GM manual showing the wiring diagrams, I just found the manual. Seems that my memory was a little off. Reviewing the diagram I noted that the primary resistance wire to the coil runs from the R terminal on the starter solenoid, then to the + terminal on the coil. The solenoid gets its power from the battery to the B terminal on the solenoid. There is a wire coming from the key switch to the R terminal on the solenoid, this wire energizes the solenoid.
If you want to PM me your phone number I'll send you a copy of the wiring diagram for your truck.
The addition of the NP540 5 spd will do nothing to improve the highway speeds for you. The transmission you have with a later model 4.10 or preferably a 3.73 and 235/75R16 tires would do wonders for you truck without braking the bank. If you could find a 5 spd OD trans, they are New Process of Clark.. I had a 72 6500 GMC for many years that had a Clark 5 spd OD trans. During my travels I have found that the 5 spd OD trans that would fit your truck were more common in school buses, especially in rural area where the bus traveled great distances.
The truck you have is on the cusp of being a HD work truck in lieu of being a highway cruiser intended for the RV trade.
