1961 Ford F-100 1946 Ford flat head V8 gauges

6volt positive ground converted to 12volts negative ground. Owner bought gauges and had them rebuilt.
Bought all kinds of reducers for 12 to 6 volts just for gauges and all the senders had been replaced.
Nothing seems to work right but a 6 volt battery right to the gauge temp works.
I tried to tell him to go back to the vdo gauges 2 1/2 and place them in the dash at 2 inch.
something like that. I do not know if the temp sender unit is wired for 6 or 12. I am guessing he went back to stock the best he could. Please forward any websites or names or info you have.
Owner wants to go back to stock the best he can. Oil, temp, gas not working at all.
3 speed transmission by the way J