Pretty sure by 56 they were 12v setups, in any case you can use a 12v battery to run the starter. Been doing that with my 51 for a couple of years now, converted everything to 12v, but the starter, it just spins faster and starts, right now. I don't have a 56 FSM, but my 55 FSM shows the P28 and P29 models are 12v. My 57 FSM shows all Plymouth's as 12v.