I was having a similar problem with the stock set up in my '57 3600. When the tank was full, it would only read 3/4. More of an annoyance than anything, but it was an easy fix.
With the tank completely full, I pulled the sending unit and carefully bent the float arm down until the gauge read just a touch over the white full dot. I did the bending in small increments, reinstalling and testing each time until I got it right. I did this with a disconnected battery and only reconnected it when I was checking it.
My gauge is now accurate. I just need to remember that empty really means empty!
If you can get to your sending unit without removing the bed, perhaps you can do something similar.
Matt
