I would say your sending unit is the culprit, and they gave you the wrong one for your application.

1980-1986 sending units should read between 80-10 ohms resistance. 10 Ohms being Full, 80 ohms being empty.

1987-Up sending units read the exact opposite. 165 ohms full, 10 ohms empty.

No the later 1987-up gauges are different.