George,
About the gas gauge, since you have checked everything outside the tank, my guess wuold be to pull out the sending unit from the tank. The last time my gas gauge did not work, the float had worked itself free from the unit, therefore giving it an empty reading no matter how much gas was in the tank. Reattaching the float to the rod fixed the problem.

On the other hand, in the 31 years that we have had the car (and as far as my memory goes!), the gas gauge only gave a very rough estimate of how much gas there is in the tank. The temp gauge, on the other hand, is "purdy" accurate.
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...and the bricks keep on rolling