The problem, and I think the confusion, stems from Kusan eventually tooling their own boxcars, not just continuing the AMT units. These are the ones I'm curious about. They are 90% the size of the proper O scale cars (the shells are 9"), with both doors cast into the body shell, one closed, one opened. These have no problems navigating O27, and apparently are pretty similar in size to the Lionel 6464's (I should have gotten into these instead of collecting odd telescopes, but noooooo...).
I know that Lionel continued the 6464 series in a sense after the MPC acquisition, and their traces are all over the place within their rolling stock. That's one of the reasons I purchased that 1979 Napierville boxcar. They seem to be a bit more abundant.
It's probably a trivial thing, but I remember being a young man and watching the Florida East Coast Railroad slogging 40 and 50 footers down the mainline, and there were a number of times where you'd have a constant string of cars, all the same height, almost the same length.
Sigh. Sorry, waxing nostalgic. Also, missing my youth. If you come across my youth, just send it back.