Bill, Andy, and Ken--I went back and figured out what I did to get my DL&W car. Using Branchline 2E Rib+3/4 rolling pin rib ends, I sanded the rectangular rib off then shortened the 10'6" end by trimming the top of the end. When I built the car, I was unaware of the prototype's flattened upper rib. I think the only photo I had at the time was the one of 52469 with the end only partially in view.
Andy, my findings are quite different from yours: the Branchline ends are an excellent match in width. I have next to me an undec IM 40799 body and Branchline 1E (rib+3/4 banana ribs)(I used up my 2E's) and they are an excellent match. Also comparing these Branchline ends to IM mini rib+3/4 ends shows matching widths.
The problem that I see is that the IM body does not have a row of rivets at the extreme end of the sides, as does the Branchline Blueprint 10'6" body. The IM body, of course, was designed to use earlier DN ends which have that rivet row modeled on each side of the end.