Gas gauge is stuck way past full mark .

I would start at the back of your dash fuel gauge to see if you are receiving a signal from the tank sending unit. Across the two leads going to the gauge, you should read some voltage with the key turned on and zero with the key turned off. If the voltage changes, the sending unit is probably OK. If not, then your problem could be in the sending unit or any of the wiring between the tank and your gauge.
Usually the sending unit is in the top of your tank and can be changed without any fuel spillage, IF you can get access.