Here's how I ran the three wire alternator on my '57 using the original harness. This might help you to sort it out.
I left the original voltage regulator as a dummy. The BAT terminal is used as a junction stud. The battery feed wire, gauge wire, large wire from the alternator and sensing wire from the alternator all attach to this terminal. (In my case, also the A/C primary wire.)
The exciter wire from the alternator is connected to the ignition side of the ignition resistor on the firewall. Since I do not have an idiot light, I soldered a diode into this wire at the alternator connector end so that the engine would turn off when the key is turned off. (If you search this site, someone recently posted that they purchased an adapter plug with the diode on eBay...plugs into the alternator, then the harness plugs into the adapter.)
Wiring as above allows the sensor wire to work as it should by sensing the load away from the alternator.
Matt
