I don't know enough about electrical items to be able to tell you how to test a coil to determine if it's an internal resistor or external resistor. I'm sure some of the electrical gurus here can tell you.

Was it originally a 6 volt truck? Maybe some one converted it to 12 and the parts books specify a 6 volt system. Sounds like you are narrowing in on it now, though. Too much juice getting to the points. So, if there is no external ballast resistor, make sure the coil has an internal. Make sure the coil voltage is correct for the truck; 6 or 12. With all of that squared away, and making sure the condensor is not directly grounded, you should be back in business.

dhoward: I have the Willys wiring diagrams.

Jeff: The truck was originally and still is a 12 volt system.

Come to think of it, shouldn't any auto parts store be able to test the coil and tell you what you have? It sounds it needs to be a 12 volt internal resistor type, what with the lack of a ballast resistor.