They were generally the same and the 56 heater should fit in the 54. Remember though that the motor in the 53-55 are 6 volt and the motor in the 56 is 12 volt (but you are probably on board with that already).
Remember also that there were two types of heaters - Fresh Aire and Hot Water. The Fresh Aire heater was ducted to the outside by a louvered vent on the cowl side in the Effies an dhas 4 controls. The hot water heater is all internal and has only two controls.