Classic/Antique Car Repair: 1955 chevy p/u, volt system, 55 chevy

Question
I have a 55 chevy pu that's been restored. They put a 350 engine in it with automatic transmission. My question is, I just purchased a fuel gauge for a 12 volt system, which is what the mech. I took it to recommended. He pulled the sending unit out of the tank and said it was a 12 volt system and that the old guage was probably a 6 volt so that's what I ordered. I put the guage in today and it worked fine. It showed I had a little over a 1/4 tank. The problem is, I went and filled it up today after I put the guage in and now the guage is gone. It went way past everything. How do I get it to read right? Do you fill it up and try to bend the guage or what? I didn't get any instructions with the guage so I just put it in and hooked it up. I don't know if there's something that needs to be adjusted or what needs to be done. Any help is appreciated.
Ronnie

Answer
Not a rhestat specialist, but if you put a 6v gauge with a 12v sending unit it is showing double what is in the tank. You can get a 12v to 6v reducer, but I'm not sure if that would work due to the grounding quirks. I would probably just pony up for a 12v gauge since the truck is not original anyway.