After you verify the ohm ranges for the new sender and your old gauge, I would pull the sending unit completely out, jump a ground wire to it, and hook up the sensing wire.

Then turn the key on and swing the arm through its range and make sure the gauge shows the same thing. If you can get full and empty on your gauge you'll need to tweak the arm to make it read right when its in the tank. This may take some trial and error to get right.

(Don't leave the key on for longer than a minute though; it may burn out the points/coil?)