Your coil needs a direct full 12V feed while starter engaged and a resisted 12V feed after start to protect the points.
I believe you need the button switch with the "side terminal" if the button switch on your starter does not have one. Easy to change and wire up.
Click
You don't even need to remove starter. Standard Motor Products SS521 Solenoid.
Two wires to coil pos. One from the ballast resistor and one from the side terminal.
You must be missing the full load 12V to coil and only have the resisted 12V to coil, which is 9V.
Starter demand, together with the fact that heat causes resistance all over the ignition system, nothing left for the points.
So advise if you have a button switch on starter with no side terminal.

ALWAYS take off the negative battery cable when working on the truck.




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