Cars built around that time employed a sender with an arm and float design similar to this:
They have a nasty habit of "spinning" so the arm reorients itself and is now in contact with one of the internal baffles of the tank. So you either get a gauge that never reads full or always stays above 1/2 or so. You got the above 1/2 scenario.
Have to drain tank and drop it and then see what's up wit the sender and if it needs to be reoriented.