[Daves55Sedan]

I would be VERY skeptical about trying to swap the sending unit with a similar part from a flathead, an early Y-block, or later year sending units if you want it to work properly with your guage.
1956 is the only year that Fomoco had 12 volts run un-regulated thru the sender to the guage.
In '57 and later years, Fomoco incorporated a regulator ahead of both gauges.
It might be possible to swap both matching gauges and sending units with a later year but then you will need to modify the left cylinder head with a bushing for the smaller design sending unit and the face of the gauges often look different from the original '56 gauges. Also would need to add the regulator and mount it under the dash near the gauges.