Power from Taillight is wired to fuel sending unit... Check!

There is no ground wire in the kick panel connector!!!!

Here's the thing............you say you have a 74 "wheel" and "the same" wiring harness. I doubt that you WANT to use a 64/5 wiring diagram!!!

Moreover, whoever hacked that thing together may have (probably did) make some mistakes, so you likely need to "dig in" and see what they spliced together in the under dash harness

The fuel sender wire should be easy as it only goes ONE place.............from the fuel sender, up through the kick panel connector, and to the sender terminal of the fuel gauge

"MyMopar" has wiring for a late "truck" and some of the "truck" forums may have info. The thing is, Ma kept the same color codes for years, but there WERE changes here and there

http://www.mymopar.com/index.php?pid=24

In the 79 diagram, the sender is Light Blue / BlacK

68-71 is Light Blue (pickups)

I'm not intimately familiar with the early clusters, but make certain the thing is grounded. This is a problem, at least on the later ones.