OK, i've fixed my problem. I had to completely retrace all the wires in the harness to find that I had crossed the wires leading to the ammeter and the ignition. Problem solved. Thanks e-one for your help.
As for your fuel gauge problem. Try this...the original Chevy wiring schematics do not show a ground between the fuel tank sending unit and the cab but the GMC one does. I had the same problem with mine until I connected a ground from one of the mounting screws to the cab. You also have to make sure the wires are connected properly at the gauge. If you cross them the gauge will never work. The hot wire from the ignition goes on the left post (as your looking at the face) and the wire from the sending unit goes on the right post. I'm not an expert but I just spent two days looking at the schematics.