Customcab,

Although they may look the same, (and I do not have a 56 here to compare to my 57) I can say that the 56 has a vertical post design as the 57-60's have a forward leaning design. The actual post cross section might be close, but depending on how much of the post you need, and/or whether you need the upper or lower portions (where the post blends into the cowl or into the roof), you would have some serious metal reshaping to do. In metal work nothing is impossible. Some of us shape patches for areas that there are no panels sold, so you probably will be able to make use of the later post for your repair.

Aloha, Tony (with a 57 styleside short box, going to the darkside)