The ground polarity is irrelevant. Touch BAT and GEN terminals on the voltage regulator.
As HRL pointed out, you can still polarize the Genny properly by running a lead directly from the battery.
If its a Positive ground system, run a jumper from the Negative terminal of the battery to the GEN terminal on the voltage regulator.
I agree with you about not taking the whole truck apart. Do it in stages. I know myself and if I disassembled an entire vehicle, it would never be put back together. I prefer driving my vehicles to working on them so in the case of Ol' Roy, I repair/restore systems one at a time while still being able to use the truck.
The truck is normally only out of commission for a few days or a couple weeks at most while I await parts or work on it.
