Jim;
I also have had problems matching the sending unit to the fuel guage. When I replaced my stock guages with Haneline guages, the factory Ford sending unit was not calibrated with the correct resistance for the new guage. I had to get a universal Haneline sending unit and cut it down to size for my Customline tank.
Also, FWIW, I worked in the HAZ MAT emergency response field when I lived in Idaho. The guys the install and remove petroleum UST's (underground storage tanks) often used dry ice to chemically inert the empty tanks of vapors before they remove them and cut them up. You can get dry ice at many supermarkets. If the tank is free from any petroleum sludge, the dry ice will force any vapors out of the tank as it goes from it's solid state to CO2 vapor.
Good Luck!
Rono
Ron Lane, Meridian, ID