This is how you mark for the holes for the moulding. You run the tape like this down the holes that are there, putting the tape down over the area where there are no holes raising or lowering the end to where it needs to be and then setting it into place and tapping it so it sticks. Of course you still need to measure height and all that from some body line or something that matches the cab. But it's the easiest way to get that line straight. You then just mark on the tape where you want to drill the holes.
You can run this tape right under the holes or through the center, I personally will run it right under the hole then mark the hole spots to drill accordingly above the tape. I do this so the drill doesn't tear up the tape when the hole is drilled with a possibility of it coming off or something.
I use to pay tape stripes on cars for a dealership 40 years ago I would do the whole side like this, one shot, lay it down then cut the tape at the door and not the clear overlay. Then go back and pull the overlay off, wham, did the whole car in literally a few minutes.
Memories, pressed between the pages of my mind.....memories.
Brian