The frames could well be the same or very similar. Frame strength is represented as RBM (radius bending moment) that takes into account the material strength of the steel or aluminum and it's section modulus, a calculation of the flange and web dimemsions. The RBM is shown in psi figures.
I doubt that 40, 50 or 60 series had much difference in frame strength on the standard specification of each series. Reinforcement of the frame was (is) a standard option on most larger trucks. You may find it interesting that F*%ds very popular F450/F550 chassis carry the same ratings that your discussing here. These are still not thought of as heavy duty regardless of what it may say on the fender badge. Same is true for Chev/GMC W4500-5500 chassis. These are actually fairly light or small medium duty trucks. True medium duties don't start until Class 6 or 20,000 GVW. Heavies begin at 33,000 +
