Looking on line for info, you are probably getting info for a GM Corporate 10 bolt, which is generally 1970 and newer; maybe not what you have in your '55.1. Go to GM Heritage website (link below) and find your truck year/model, where you can download the correct "vehicle information kit". That will show you where to find you rear axle casting numbers, which will tell you what rear axle you have, assuming it's original. If not original, then look online for GM 10 bolt identification charts and go from there.
https://www.gm.com/heritage/archive/vehicle-information-kitsThis link has service manual info which could help identify what you have also.
https://chevy.oldcarmanualproject.com/
