Is the sender the correct ohm value for you stock gauge? The factory fuel gauge system is really simple. B+ to the gauge on one side and the sender sense wire on the other. The sender has the sense wire and a local ground. Did you ground it to the chassis or just the tank? Did you extend the original sense wire back to your new sender or did you just tap into it. Are you using a factory service manual schematic or something simplified?
Vette Vet is really good with these and here is a thread he posted to that should be very helpful.
http://67-72chevytrucks.com/vboard/s...d.php?t=327643