Fuel sender and gas gauge in ‘63 and ‘64 Darts.

I would try to save the original sender unless it's bad, I would remove it and test it and if it is good I would replace the sock and the float. The brass float is still available from most Ford resto vendors if you can't find one from a Mopar vendor. I would also check the IVR and make sure it's operating within spec, and I would also drop and clean the tank, so you don't have issues with a dirty tank.