Is your amp meter reading a slight draw (negative) sometimes or every time you turn the ignition off? I had a similar problem with my '48 before and after my new wiring harness. In the end my trouble was caused by the 'cut-off' on the Voltage Regulator.
Like many have suggested their are a variety of possibilities, at least we are better acquainted with our trucks after chasing down these kinds of pain in the [censored] problems...
Good Luck
