It's quite easy to go from 6v to 12v. All you need is a 12v alternator, a 12v coil and some resistors for any electrical gauges, heater motor, wipers etc. and changing all bulbs to 12v.
I've done 2 of my old cars and am currently doing a '46 Ford. I use the 1 wire Delco alternators. The points ALWAYS run on 6v, so you need a 12v coil with an external resistor or I use the NAPA coil # IC14SB which has the resistor built in. It just looks cleaner to not have a Chevy type resistor bolted to the firewall.
You will need to put a dropping resistor on the fuel gauge and any other electric gauge. Many types are available. You also need to change all your 6v bulbs to 12v equivalents.
There is no need to change any of the wiring unless it is bad to begin with. Also, no need to change the starter or solenoid. The engine will crank and start so fast with 12v going to the 6v starter you will never overheat it.
There's a guy with an eBay store called Kansas Select Products that sells anything you need for the conversion. I've bought things from him for my cars and he's great to deal with and very helpful if you have problems.
Google "6v to 12v conversion" and you will be reading the rest of the day!!! There's TONS of info on the internet.
Steve,...Speedway Motors catalog has 2 kinds of resisters,one for gauges and the other is for motors (wipers ,heater etc.).They have lots of street rod parts,I have been pleased with there products and service....Jim
In my opinion, the all-time guru on voltage conversions is Randy Rundle of Fifth Avenue Antique Auto Parts. His website is fifthaveinternetgarage.com or Google "Fifth Avenue". Randy sells conversion goodies and has a pamphlet for sale on the subject as well. His site is very interesting, even if you're not contemplating a voltage conversion.