Having done a few changeovers to the 605 Saginaw box,there is a much better and stronger unit available,it is a 500 box,you may want to look into investing your $$ in one.

the factory 605 gear box has to be modified to fit 55 to 57 chevys also the steering column shaft is 1 piece on 55-57 .the 605 box has a seperate shaft .to get an idea of what you are looking for google classicchevy international.it will take you to a parts catalog.find steering ,then steering gears.will show gear boxes made for the conversion.