Captain,
Sounds like you might have a 90 ohm sender. That would make the gage read full with just a partial tank. Our trucks us a 30 ohm sender.
Check out these threads.
http://67-72chevytrucks.com/vboard/s...d.php?t=432957
http://67-72chevytrucks.com/vboard/s...d.php?t=413579
TR