Update- Yesterday I found out that little terminal under the fuel terminal is only "hot" with the key on so I tried hooking up the gas tank wire to it and the gas guage went up waaaay past full. So I put the wire back to the correct terminal and the gauge went back down to 1/8 even though it has almost 10 gallons in it. I touched the fuel terminal with the test light and that was the ONLY circuit where the light was dim. That makes sense since you said it was only about 5 volts. This leads me to believe my brand new sending unit might not be so accurate.

. As soon as I get the brake lights fixed we will take it on it's maiden voyage to the gas station and fill it up and see what happens.