Weird Fuel Gauge Problem after installing new harness

[rotorwrench]

I'm not sure if your harness included the wire from the indicator back to the sending unit. The power circuit from switch to indicator should be in that harness you indicated. If it has an original King Seeley sender is also an unknown. A lot of folks use a new tank due to corrosion with a modern replacement Ohm controlled sender unit so it can make a difference.