Frame hole Measurement help

[Flashy2003]

I have a 68 Short bed C10. Originally the truck was a step side but I am going to a new fleet side. The problem I have is the frame does not have the holes for the rear cross member. Can anyone help me out with some measurements? I don't have the bed sides yet but I would like to get the holes drilled so I can start mocking up the bed floor. I would like some measurement from the rear of the frame to the front and back of the hole,some measurements from the inner and outer of the frame and a measurement from a reference point on where it belongs from another hole or mount in the frame. A picture with a tape measure across the area both ways is all I really need. It would be really appreciated.