Bummer! I'm not big on any of the AMC cars, but Gremlins were the worst.
Seriously though, my experience has been that the sending units tend to be intermittent and quit sending at spots in their range. So my testing would start by grounding the sending unit wire near the tank and checking to see that the gauge goes to full scale. If so, that proves the wiring to that point and all you have left is the connection to the sending unit, the sending unit itself, and its ground.
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI