I’m wondering if you have the correct sender for gauges. According to the offerings at Classic Parts, there’s one sender for gauges and a different part for idiot lights. If you’re running the off/on type for lights that might produce the full hot reading. Here’s a photo and link for the gauge sender, followed by the sender for the lights. Also make sure you have no teflon tape on the sender threads which can insulate the circuit and ruin the reading.
http://www.classicparts.com/1956-72-...ctinfo/53-811/
http://www.classicparts.com/1961-66-...ctinfo/53-812/