58 and 57 Frames the same

[BadDaddy]

Ok, I'm brand new to this site. 30+ years ago I saw a 1958 1/2 Napco 4x4 shortbed at a car show and knew I had to have one. Now my 15 years old son and I are going to build one as a father and son project. The budget doesn't allow purchasing a complete truck so we're starting with a bunch of parts I acquired from a guy that was also going to build one. My question is this: The body pieces that I got are from a 58 but the guy thinks the frame is from a 57. I see a difference in the specification manuals of about three inches measured from the center of the front wheel to the edge of the bumper (57 being shorter.) Is the 57 frame actually shorter and if so is this only an issue in regard to mounting a bumper? Thanks for the help. John