I believe your stock oil gauge is mechanical. If the copper tubing is crimped or broken you can get a nylon replacement tube and brass fittings from NAPA. The mechanical water temp gauge is another story, it's a closed tube with some kind of extremely-low-boiling fluid inside. Once you've "let the smoke out" of the tube it's done. Some early GMC water temp gauges are electric, if this is the case you got lucky. The fuel gauge should work fine as long as you have a resistor on the positive terminal for the 12v conversion. I'm assuming it's been changed to negative ground if that was necessary.
