Be safe and follow the wire diagrams posted on the forum threads at the top of the Classic tech forum.
You will have one wire running from your constant voltage regulator (same as wth oil gauge) to one side of your fuel gauge. The other side of the fuel gauge is connected to a wire that runs all the way back to you fuel tank to the sending unit.
New under dash harnesses are expensive. If you do buy one, make sure it is the one made by "Alloy Metal Products". I've had to help friends correct wiring mistakes on the ones made by Mr. Mustang. You can also search ebay for a used one.
You might be able to get a "refurbished one" from Randy at Mid life Harness restorations. here's his contact info
Midlife Harness Restorations
2316 E. 9th St.
Lynn Haven, FL 32444