I'm going through exactly the same process right now. Manual says Magneti Marelli 275, but I can't find any cross references to that number in current day cross reference charts.
Mine is currently fitted with a champion L82YC. The L corrseponds to a 14mm thread 0.5" deep, which seems to correspond to my thread depth in the head. The YC is standard projected nose and a copper core. The 82 is the heat range. My plug seems a little dark in colour (Milk chocolate brown) which may indicate its running a bit rich, but the jets are standard in the carb, or that the plug is a little cold.
Anyone have any thoughts or know what the correct plug should be for a 1957 175CSTL?