Your best bet in the long run would be to switch your truck over to 12 volts. The sad fact is the positive benefit we get from 12 volts ( that it takes about half as many amps to power the same accessory on 12 volts) as the original 6 volt part... works just the opposite when you go the ther way. The compressor that requires 8 amps on 12 volts would require about 16 amps on 6 volts. Due to things like electrical friction and related... the compressor will likely require close to 20 amps...not a very efficent trade.
By converting to 12-volts your truck will start easier (especially with the a/c on) and you will be able to install more of the modern accessories like a radio and related. Thanks Randy
