Long or short? As far as I know, the step side boxes were (almost?) identical between the two era's and all the way up to 87 I think.
60-87 stepsides the same?...from what I can recall.
I think I also recall that those stepsides and fleetsides will interchange with no hassle. You put it on and bolt it down. Please correct me if i'm wrong (can't stress that enough). Good Luck.