The generator should tell you all you need to know, someone may have put the wrong battery in it and that's shy it got parked?

About the only 6 volt things that would not survive 12 volts very long are light bulbs (seconds of life, if that), and the fuel gauge (a few minutes). Longer term, a 6 volt wiper motor won't hold up to 10 - 20 manures of constant operation (they run really fast until the smoke fills the truck cab). Starters and charging system will fail eventually (depending on their original condition). I may have missed a few things, but you get the drift.

My son and I cruised a 48 Dodge (definitely 6 volt system) with a 12 volt battery up and down the road in front of our house for many hours (for about 10 minute streak each time) over a period of about a month with a 12 volt battery strapped in.

Some information sources state the 6 volt to 12 volt conversion in the Dodge truck division occurred between either the 54 to 55 model year or the 55 to 56 model year, or the 56 to 57 model year. I guess you get to pick your time frame.

Been running a 51 Plymouth on 12v for about 2 years now.

I forgot about the wipers, mine are electric and I built a current limiter to slow down the wiper motor. Poorboy is correct about the wipers working real fast on 12v. I am not sure which would happen first, smoke or the linkage getting beat to death. Obviously, anyone with vacuum wipers doesn't have to worry.

As for the starter, been running that on 12 just as well. No issues. I talked to a local starter shop about converting it to 12. According to them, that can be done, but you don;t need it as long as the engine starts easy. Mine does. If I eer burn it out I'll spring for the conversion, vintage powerwagons may still sell a 12v starter. These guys make a modern ministarter for it

https://snap-usa.com/products/1950-1955-dodge-plymouth-mini-12-volt-starter-replaces-mch6101

As for the fuel gauge, a runtz resistor (it's not really a resistor) will take care of that, the rest of the gauges are mechanical except the ammeter and it don't care about voltage. If I was just going to spin it over and see if it started I'd use a 12 battery.

To be honest I would give the wiring harness a real close look, odds are if it's original it is falling apart as we speak. If it is I wouldn't use it. I'd disconnect it and run a jumper wire or two for testing the engine.

I had a 54 Dodge pickup, it was still 6 volts. I got away with the 6 volt wiper motor for a couple of years! I'd sort of use it like a time delay wiper, I hit the on switch for a few strokes then shut it off. If you got in a bind it actually worked OK (but was really fast) for about 5 minutes. My buddy and I got caught in a down pour once, even with the wipers on hyper speed, I could barely see the road. we were in an area we didn't know at all and was looking for a place to pull over. We finally found a spot to pull over but the motor went up in smoke before I got it shut off. We had to open the windows because the cab was full of that nasty burnt electric motor smelling smoke. It was so thick we couldn't see across the cab! It was still pouring outside, so me and my buddy both got pretty wet. We had to wait for the smoke to clear the cab, and we had to wait until the rain slowed down enough I could see the road again. We were fortunate that the rain was pretty well past us, so we made it home without wipers. I managed to cobble in some sort of modern wiper motor, and my buddy and I made several runs with that old Dodge. Everything we got caught in a rain, my buddy asked if we were going to see the wiper motor go up in smoke again and we both laughed. Good times! I put 40,000 miles on that truck in 4 years time, and it sat 3-4 months during the winters!