Thanks to TBONE1964 and his post
http://67-72chevytrucks.com/vboard/s...d.php?t=640615 , I may have this figured out. I hope so, because my '72 C20 has the same problem. Here goes...
One of these things has probably happened:
1) The gauge has failed.
2) The tan wire is not connected at the gauge, or the connection is corroded.
3) The tan wire is open, that is, it has a break in it. You had to handle it while installing the new tank, so this is a likely possibility.
You can check for all of the above by measuring voltage to ground at the terminal on the sender that's connected to the tan wire (measure with the engine running). I'm not sure what the voltage should be, but if it's more than zero then none of the above are the cause of the problem.
4) The tan wire is disconnected from the sender. You can check this visually, but just to make sure there is no corrosion on the tan wire terminal, clean the terminal. The best fast way is just remove and replace the tan wire connector from it's tab several times.
5) If the gauge needle still reads 3:00, it's almost certain that the ground wire from the tank sender to sheet metal is the problem. Run a aligator-clip jumper from the ground tab on the sender to a clean, paint- and rust-free spot on the sheet metal. If the needle drops down to a reasonable level, your ground wire is the problem; repair / replace / re-terminate / whatever as required. This is the most likely problem, because grounds are almost always a problem.
Good luck and let us know what you find, especially if my suggestios don't lead to a solution.