The straps on your tank, 71SSCLONE are correct. The small hole, facing the bumper is supposed to have a self tapping bolt going threw it into the shorter part. Then bend the strap slightly at the back of the tank. Tighten the long bolts near the rear differential till the straps are tight against the tank.
As far as your gauge, it should read full unplugged. Check the ground wire for connection and good ground. If that is good it is the rehastat. Basically the sending unit is no good.