Hy CRL3100, unfortunately there are two different 1955 truck models, one six volt, referred to as an "A" or first series, and a twelve volt one, referred to as a "B" or second series. If the ring gear is for a six volt system then you will be limited to an early fifties car starter or a starter from an automatic transmission equipped truck manufactured up to the 55 "A" or first series. If the ring gear is for a twelve volt system then a starter from a 55 "B" or second series or newer would work. I believe the tooth count for the six volt ring gear is 139 teeth and the twelve volt ring gear has more teeth than that ( I can't remember the tooth count at the moment) hope that helps.
