Ross,
I don't understand how the starter post could be dead but both side of the fuse, one side of which connects to the starter post, could be live.
Otherwise, assuming I'm confused about what the "post" is, it sounds like a bad connection at one of the ammeter terminals. Quick check is to put a jumper wire across the two junction box terminals. If you're using conventional Model A wiring, that will jump you around the ammeter. This assumes that you're using the very early "A" wiring in which the ignition circuit didn't go through the ammeter. If you're using the later, perferred, wiring, then I really don't understand how the car could run.
Steve