The best way to do the conversion is to throw away all the old wire and put in an aftermarket kit. Then you have fuses for everything. I did just that on my 54 Ford. Its the best move. All your switches (if they are in good condition) will run fine on 12 volts - headlight, ignition, heater blower, etc. Lighter will have to be changed as you already found out. Heater motor will have to be changed or you will need a voltage drop. Get a blower motor for a 56 - I assume they are the same. For your guages get one of those voltage drops Ford used from 57 into the 70's/80's (these are for gauges only-do not use anywhere else). Go to an alternator with integral regulator. You don't need to do anything to the starter or distributor. Need a 12 volt coil, make sure you use a resistor. Your only problem may be the radio - some won't take to a change in polarity and you will need a voltage drop for it. Change all bulbs to their 12V equivilent. I probably forgot something but this is a start.