The Thunderbird line came at a unique time in the evolution of Ford cars.
1955 T-birds were fitted with six volts and positive ground. (all gauges and devices were six volts, and had been for many prior years).
1956 T-birds changed to 12-volts and negative ground, BUT all the gauges were also changed to 12-volts. The same happened across all lines of Ford cars and trucks. This is the ONLY year Ford used 12-volt gauges.
1957 T-birds retained 12-volt/neg gnd BUT the gauges went back to six volts. Again, this happened across all car and truck lines.
For some devices, this swap is easy. Not so for everything. Stepping down voltage for gauges can be done with a good constant voltage regulator under the dash. If a 12-volt generator is used with a 12-volt battery, the charging system's voltage regulator must be swapped as well.
The only difference in the starter motor and generator is the field windings. As stated, the starter motor simply runs faster which tends to make the engine start sooner. A shorter duty cycle keeps heat down, so no change is necessary. The clock is a solenoid-operated device that needs no alteration.
Permanent magnet motors will run backwards and a lot faster. Since we have used resistors for speed control successfully, the heater motor leads need to be reversed with the addition of a modern heater motor resistor for speed control.
Change all bulbs to 12-volt including the dash (especially the GEN light) and radio lights, dome, glove box, stop, signal, running, backup, headlights, etc.
The radio needs to be retrofit to 12-volts OR all the tubes must be changed to their 12-volt counterparts and the '55 radio vibrator needs to be changed. Gary Tayman Electrical
does a beautiful job of retrofitting these radios, giving them AM FM AUX stereo with left-right and front-back fade. He did two radios for me and I'm happy because the radios look BONE STOCK. His number in Sarasota, FL is (914) 317-8924. Gary is also a T-bird owner.
Each gauge has two posts. I would add a constant voltage regulator and reverse the polarity of both gauges. When done this way, the original sending unit may be retained.
It's easy to understand the need to standardize across car accessory lines but Ford made major electrical changes in '55 thru '57. The next change came with fitting alternators because of higher current demand. It continues today with electric fans and electric water pumps that run when only necessary for more efficient and gas-saving heat/cool control.