76 k100 won't shut off

Do you have a series/parallel switch?

 

I would think it would be a 12v. Soleniod..
In cause your not familiar with how it should work it takes power to activate the plunger to open to let fuel flow into the pump when you shut the Ignation key off that shuts power off at the soleniod shutting the plunger which in turn stops fuel flow to the pump shutting the engine down
The reason @Oxbow asked of you have a parallel switch is because of the vintage of your truck at that time Pacar and some other manufacturers were using a 24/12 volt system which meant the starting circuit was on 24v but most everything else on the truck was on 12v.. chances are at this point in life it has been updated to just a 12v system.. reguardless I think that fuel solenoid was always 12v ..

 

Just to add to what @Oxbow is asking, your battery box look anything like this? This is my 83 K100C….

 

I try to distance myself from Cummins enough that I can't steer you in any certain direction
I would surely think you could find a after market soleniod some where .. I would try my local NAPA 1st
@OLDSKOOLERnWV is a wealth of Knowledge on Cummins and where to get the best parts