I'd think a 376 is the correct type from 54' (and the better carb), but the 276 will work fine, and looks good (vintage) with the external float. (edit:as stated in posts below, 276 for all plungers according to Bacon books).
Whichever you go for, find one in decent shape (flooding, leaks, worn slides, wrong\worn jets, bent flanges at head intake) and don't forget to get the right diameter\type for the bike. Evaluate if a brand new one could turn out to be the better deal (and less trouble).
(edit 2: used a 376 for a period, tight fit but no problems, bellmouth. Clearly the better carb, much better float system and overall behavior, but reverted to 276 for the looks (and the leaks :O)
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