If you swap to an alternator, you will need a 12V overdrive solenoid. The unit now in the car is meant for 6V applications and will work with 12V but will burn out eventually. Most later Ford and Studebaker 12V solenoids will work. Just make sure the mount holes and wiring connections are the same.
Most folks who go to a 12V setup use a GM one wire alternator. Also, don't forget that you have alot to consider when swapping to 12V...things like bulbs, gauges, coil, etc. And, the old, brittle cloth covered wiring should be inspected and replaced as needed.