I had to go with a programable gas gauge with my 81 as it's not the same as any other manufacture. think it was 13ohms empty and 127ohm's full.
I was going to use an aftermarket float to match up with the GM calibrated gauge but I couldn't use it in the tank as the float arm would hit the top of the tank when it was only 1/2 full.Just pull the sender and hook up a ohm meter to it and check empty and full when you remove the tank.