The bed itself is the same from '55 2nd-'87 for the same length box. There are differences in th bed mounting, # of boards, running board and fender locations, but the basic box is the same. I'm thinking with the '60-'66 version, the '60-'62 are the same, then '63-'66 are the same. You might check out Mar-Ks site for more details, there is a chart there with a breakdown on the differences in the boxes over the years.
