Sounds like you are pulling the relay in with the Start signal from the Ign switch, but are lacking the run circuit voltage to keep it fired...
You sure you are not feeding the relay from the old feed wire from the Starter R terminal on the solenoid?
The R terminal receives power only when the key is in the Start position. Once the key is released, the R terminal loses its power supply.
Feed the relay voltage supply from the Run terminal on the Ignition switch...
It should have 12V when Ign switch is On, and 0 V when Key is off....
If this truck had a Coil and Points style distributor as original, it would have a resistor style wire in the original coil feed circuit...dont use that wire to feed the relay...it will not supply enough voltage to the controller.
Hope this helps...Bill
