The ignition switch is not ground polarity or voltage sensitive. No changes are needed on the switch. This goes for the pillar light switch,headlight switch (The headlight switch with a dash light rheostat will not dim enough on 12 Volts but the switch type is fine), starter push button, and horn button. The stock 49-50 heater switch will sorta work due to the resistor mounted on the back of the switch--low speed will be nearly the same as high speed. You will need to reverse the stock Ford 6 Volt ignition coil primary connections and add a resistor in series with the wire from the ignition switch to the coil DIST terminal. (Note: most replacement "12 Volt replacement" coils require the resistor but not the primary wire reversals). Replace all bulbs and lamps with the 12 Volt equivalents. Place reducers on the oil, fuel and temp gauges. Re use horn relay, starter relay, and add a resistor for the stock AM radio. You can use the 6 Volt starter until it fails if the engine is stock (original 6 or 8 cylinder). It will be easier to find a 12 Volt heater blower motor than to use a resistor for the motor high speed and also replacing the low speed resistor on the back of the blower switch. (You may want to find a 3 position pull/push switch from a 1951 and have a 2 speed heater with a replacement 12 Volt 2 speed CCW rotation blower motor). Going 12 Volt requires a 12 Volt generator and regulator or a 12 Volt alternator and regulator or a modern 12 Volt alternator with internal regulator and of course a 12 Volt battery. 12 Volt negative ground will require reversing the ammeter wire through the loop for proper indication of charge/discharge. No voltage drop needed for the ammeter. If you have overdrive you will need a 12 Volt solenoid and relay. If you have turn signals you will need a 12 Volt flasher unit.
I changed my 50 over to 12v neg ground about 14 to 15 years ago, I used the gm style alternator at first but didn't like it. I changed to a mid sixties ford alternator with external regulator, it looks more like it belongs. As a personal preference I changed to a starter solenoid that is isolated. With a isolated solenoid I wired positive through the ign switch and neg through the starter button, this way the key switch has to be on to engage the starter. I'm still using the original starter and have not had a problem.
thanks guys for all your input so far so good i used the gm style alternator changed all the bulbs, have to change the direction of the ammeter loop(tomorrow) left the 6v horns sounds like freight train LOL