Goofy Gas Gauge '82 C-10

[Bacca]

I'm no electrical wiz so I'm easily stumped. So I'm looking for advice.

PROBLEM: 1982 C-10 that the prior owner just cut every corner possible on any repairs he did. 1st Gas gauge read half to empty only. Tank had a leak so when I replaced that I replaced the sending unit. Gauge still read the same. Obtained a used Gauge from a member here that I was told was in good working condition. Installed that one. It now reads over full by 1/4 tank at least and doesn't appear to be dropping with truck use.

A couple of variables that may be playing into this:
I believe the truck was originally a 6 cyl. It now has a "newer" V-8 with an A/Market electric fuel pump and the motor has an electronic ignition with a carburetor. The wiring to the gauge appears to be fine.

I know tough to diagnose on-line but I thought I'd throw it out there.
Thanks Bacca