This may appear a mite crude, but the measurements are accurate. You can base it all from the rocker panel and quarter panel to locate the channel (and the moulding) accurately. I've included a pic of the rearmost mounting bracket (channel) hole; all of the bracket holes are .500" (hole cneterline) below the top edge of the rocker - the rearmost bracket hole is 2.125" forward of the quarter panel edge to the hole centerline. Rear moulding screw hole is 2.500" forward of the quarter edge to the centerline of the screw hole. Installation should be pretty straightforward - bracket locates entirely on the rocker, clip the moulding in place, check the alignment for parallel to the rocker, locate the rear moulding hole 2.500" from the rear qtr. edge, and then drill the last two (front and rear) moulding holes to finish. You can put a dab of paint or sealer on the moulding screws to protect the metal after drilling. Mock it up using these dimensions and see if I got it right.
Be sure and compare the old moulding to a new one if you have one - if you're using a repro, check to make sure all of the bracket holes and the overall bracket dimensions are a match - we've all been burned by poor quality reproductions. If you need addtional pics or info, let me know. I have several sets of NOS if you need dimensional checks -
Regards,
Steve