Sender at the top of tank and fuel line from the bottom of tank? 47-53
Or, sender and fuel-line out the top in the same unit? 54-55 (and later)

12v vs 6v does not matter with a sender - the ohms are important.
Original was 30 ohms (years later the ohms was 90).

1950 tank:
http://www.classicparts.com/1947-53...l-Style/productinfo/23-523/#.Vc2-DJ1Viko