Ohms/Resistance
(Empty/Full)
Found this on a different site, if it's correct it could explain the backward reading if you used the explorer instrument cluster. I would say you have three choices, 1) use is , fill the tank before it gets to the full mark. 2) replace the gas gauge in the cluster to the bII gauge, or 3) replace the sending unit to an explorer one which would need modifying to work right due to different tank size. Haven't seen any comments about this in the swap acticles so not sure why you are having the problems with it.
0-90 Ohms most GM cars, 1965-up
73-10 Ohms pre-1989 Fords & most Chryslers
240-33.5 Ohms Industry standard, works on many popular cars
0-30 Ohms most pre-1965 GM cars
16-158 Ohms most '89-up Fords