So I am guessing that the 3rd cross sill description from Mar-K refers to the 3100 case and probably means the 4th cross sill on the 3600. Given the difference in frame width as you go back from the cab between the two I suspect that the 3100 frame requires the offset blocks to get the holes in line with the bed which is straight front to back. On the 3600 that lubersawyer and I have the offset is not needed but can be used without a problem.

1949/50 3600 Project