If you're going to kitbash anyway I'd buy some hitches, MT used to sell theirs as parts, some plastic strips, and some MT, IM, or Athearn flatcars. There were so many variations that you can basically make it up as you go. Just find a picture of one you like and start in.
The BN still ran some early "50 foot" TOFC flats in the early 70s, so they lasted well into the 89 ft era.
