The technical explanation here:-

http://www.studebaker-info.org/tech/6-12V/6-12-6.html

I'm sure other people will be able to recommend the make of resistor to buy.

Location: Oxfordshire

Lots of choices ! Look at the Cornwall Club site. Measure the resistance of the fuel gauge. Probably 30 ohms..Use a series 30 ohm resistor. Dissipation will be <1.5 watts so 2.5w or 5 watt ample.
Technically motors are a little different. Resistors will reduce the start up torque but it does seem to work. 2ohm is appropriate for a 6v, 3amp motor and I've used one in my Ruby. However in my PD I used a "buck" or step down DC/DC converter from eBay. search for 12v to 6v buck or step down. The one I've pictured is easy to mount. There are plenty of open pcb ones that are rated at 8 amps and more.

Location: Cornwall

I lost my original uncompleted post(above) and retyped hurriedly. So missed out:
Cornwall site: http://www.austin7.org/Technical%20Articles/Conversion%20to%2012%20Volt/
Resistor rating for wiper motor 2 ohms 25watt or greater.
Electronic regulators draw a small permanent cuurent so will flatten the battery in long term storage. Switch before the regulator or use a Battery isolation switch.

Location: Cornwall