Sorry, have been traveling today. But you figured it out - you have the wrong sending unit. The Bullnose senders have ~12 ohms for Full and 72 ohms for Empty. The later sending units are reversed and have a different range, like 22 ohms for Empty and 120 ohms for Full.
You may have a problem as the sending units and tanks are keyed, with the key on the sending unit and the slot on the tank. So if the later sending unit fit then you probably have a later tank, or someone cut new slots or cut the key off.
A way around it is to use a
MeterMatch, which does allow you to match the sender to the gauge. I'm running one and it works.
Gary, AKA "Gary fellow":
Profile
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
Locked
.