It's very obvious you need to find someone with a basic knowledge of DC electricity to help you out. Your comment about shorting out the generator to "test" it speaks volumes about your lack of understanding of how to work on an electrical system. The only things that really MUST be changed to make your truck work on 12 volts are the coil, and the charging system. An alternator is a much better choice than a generator, but if you want it to look close to original, a 12 volt generator and regulator can be substituied for the 6 volt components.
The fuel gauge will need a voltage-dropping resistor installed inline with it. Other things, like headlights, tail light bulbs, instrument lights, etc. can be changed to 12 volt items. If you choose to run a radio, there are lots of choices of 12-volt items that will work well and look OK.
Try to find some expert help to get the system up to par, before you end up with a serious short, and possibly an electrical fire that will destroy your project!
Jerry
