hmmmm, just out of curiousity, is there room for the needle to sweep back around the other way?
Can you get a pic of it?
This may or may not help, but when I tested a cluster in my '93 I pulled the cover off of mine and cleaned the guage faces and back side of the cover. I didn't realize it untill I put it all back to gather that my speedo was all the way past 120 as far as it could go and just sat there. even powering it on and making the guages sweep had no effect. I ended up just going for a little drive and as soon as I hit about 40 MPH, the needle jumped back and acted normal as if nothing had ever happened.
I am wondering if maybe something like that happened here. If there is enough room for the needle to sweep back the other way around, I personally would try to drive for a while and get the sender to sweep full to empty then fill the tank back up.
Just a thought. Might not work at all, but IMO, it is worth a try before dropping the tank.
1982 Cutlass Supreme - mild SBC, tons of fun! The summer toy
1996 Silverado Z71 - the hauler
2002 Bonneville SLE - His / project
2003 Bonneville SSEi - Hers / never ending project black mirror