The high reading may mean the gauge is reading a high impedance through
the sending unit, or that the ground wire at the tank sender doesn't
have a clean (zero-ohm impedance) connection at either the tank end or
the frame ground end. I'd take it back and ask them to check the wiring
connections at the tank to ensure they are correct, tight, and clean.
Obviously if this problem just started immediately after the work was
done, they may have messed something up. While there, they could also
check the sender itself (wires disconnected) by attaching an ohm meter
across the two non-ground connectors and manipulating the tank float
through it's full travel (I think they can do this, I'm not familiar
with the gas filler and tank configuration on your year Corvette, but I
think it was nearly the same as yours all the way back to 1963 except
for the addition of a vapor return line)... impedance should smoothly
transition from about zero ohms (float all the way down) to about 90
ohms (float all the way up).