The dimensions are the same on those trucks as far as the bed goes, which would tell me the frames are pretty much the same also.
I know the sheet metal design is different cosmetically, however if you were to replace the wooden floors on late 60 -72 trucks the length and widths of the boards used are the same for those years.
I imagine you could use the article for the 66 you were talking about and apply it to your 67-72 truck.
You could also check with some one like www.earlyclassic.com or www.gmcpauls.com. They would know for sure.