I have a 68 Coupe which has a standard fuel gauge that reads wrong. For example, when the tank is full, the needle points way past full, and when the tank is near empty, the gauge points just past half-way. Is this a sender or a gauge issue? Any advice would be greatly appreciated.

Most likely a poor connection, either from the gauge to the sender or the sender ground. GM gas gauges use 0-90 ohms, and there is about 45 extra ohms in your circuit. Try cleaning the ground connection first, and make sure it is bolted tight to clean bare metal.

I had a similar problem, I filled the gas tank so I knew it was full and repositioned the needle on the guage. Haven't had a problem since.