I used a fuel pump from a 39 Plymouth motor in my 38 Plymouth and it worked just fine.

I would think either pump would work, the only issue I've had with fuel pumps is the placement of the sediment bowl. Some years, the bowl was too low and conflicted with my splash pan.

Does anyone know if a 1938 fuel pump will work on a 1939 Plymouth?  Bolt pattern is the same, the gasket is the same, but will the part that goes in the motor work if it has a curve instead of straight arm.