You should have 2 wires going to your current sender. Light green for cold and dark green for hot. Both lights share the same pink wire (as do other gauges/lights). Looking at a wiring diagram, I'd say you could cap off the "Cold" light wires and use the "Hot" light wires for your gauge. Pink going to right terminal (view looking at back of gauge) and dark green (dark green typically used on 60's trucks w/ gauge)connected to left terminal and then on out to sender. The same gauge wire is used for lights and gauges so you'll be fine.
-David
