The tan 20 gauge wire DOES go directly to the fuse box on the '67 through '72 trucks! From the same terminal, another tan wire goes to the fuel gauge in the instrument cluster. The other side of the "Gauge Feed" is pink, 20 gauge and goes back to the fuse box. It sounds like you do have the correct wire as "Tan" has been the fuel wire color at least since 1956.
Wires "going to the fuse box" does NOT mean that they are fused! On the '67 -'72s the tan wire is just a "Feed Through," a way to plug in the wire. I really doubt there is much difference in a 1966 and a 1967, but there could be as the truck body changed at that year break!
The above info comes from the 1967 - 1972 Master Wiring Diagram from Chevrolet. If the wire was from an auxiliary tank, there would be two tan wires with a switch before the gauge.
There are lots of reasons why a gauge is inoperative. Open connections, bad sender, and bad gauge are some of them. Start at the 20 gauge pink going from the fuse box to the panel and trouble shoot from there.
best wishes,
Les
