These rebuilds all had the preheater arranged in two barrels placed on the sides of the boiler; this arrangement had the disadvantage of subsequently increasing the axle load (therefore restricting route availability) and stretching the loading gauge to the limit, and precluded similar attempts from interested parties such as the British Railways.

[1] An alternative arrangement had been experimented by the Deutsche Bundesbahn in 1952 on two DRB Class 42 locomotives, in which the barrels were placed below the boiler, which had been appropriately raised.

To accede to BR's entreaties, the Società Franco studied a similar concept to transform a Class 740 with a single preheater barrel, and a single chimney placed on the right side of the locomotive, to test the concept (which would eventually lead to the ten Franco-Crosti BR Standard Class 9F locomotives).

To keep down the cost of transformations, the boiler was left largely unmodified, and only an exhaust steam injector was fitted.

[2][3] The first locomotive was outshopped from the Verona Porta Vescovo workshop in 1954, and was originally classified as Class 743.433; tests showed that the locomotive's performance was practically the same of the two-barreled Franco-Crosti 743, having a power output of 1,100 CV (809 kW; 1,080 hp) at 45 km/h (28 mph), to the Class 740's 980 CV (721 kW; 967 hp) at the same speed, while enjoying a greater route availability.