OK- - - -have you used a jumper wire from the battery cable terminal at the starter to the small terminal on the engine side of the solenoid cover? That's the circuit the relay is trying to complete. If the starter functions that way, any magnetic start switch will fix the problem- - - -even one for a (GASP!) Ford or MOPAR. You don't need the original equipment relay. I assume the truck is still 6 volt? If so, check somewhere like Rock Auto or NAPA for a 6 volt starter relay for an early 1950's Dodge, Chrysler, or Plymouth. 6 volt Ford starter relays use a circuit that is grounded to engage it, while MOPAR relays use applied voltage to energize them.
Jerry
