Thanks to member JetMech, an awesome member by the way, I've got pictures of the mounting area for the cargo light with measurements. There are just a few things that need to be noted.
1 - The above drawing, indicating that the center line of the square hole is on the center line of the cab is WRONG.
The 2 smaller round holes are an equal distance apart, their center line is on the the center line of the cab. The square hole, indicated by the new pictures, is OFFSET to the LEFT slightly.
2 - The sheet metal roof panel meets the sheet metal panel of the rear at the cab, at this meeting place, there is a small "valley" where the 2 sections are mounted together. Any measurements in the pictures that go to this "valley" were measured to the top edge of the ROOF side sheet metal, NOT the rear cab sheet metal or the middle of the "valley".
The new pictures are soon to come.