been a while since I fooled with a balast resistor, but ya, nicrome wire in ceramic is the way I remember it. so.... 2-3 amps of coil current and a 2 volt drop across the resistor is 4-6 watts. not all that much heat.
yes, I had a couple trucks with a dual post battery... we even had a company up here that made them. we also used to tap a screw into the cell connector between 3rd & 4th cells to run the gauges. pretty much everything else was easy enough to convert. I even had a positive ground GMC truck once