This truck has an Impco fuel system with an EPR (electronic pressure regulator). It has no injectors. My guess is the lock off valve is stuck in the open position causing raw fuel to dump into the epr and then out to the mixer. You can take the lock off apart and clean the valve. (I find gasoline works well). If you do that and it still leaks through the EPR, then I'd replace the lock off and EPR.