Pingman -
Thanks for posting that drawing. It's the same one that appears in the book I referenced. It shows the main roof height at 13',6-1/4" and the extreme height over the dome as 15'-10", but, as I noted above, it doesn't give the rear roof height.
I haven't bothered to check the dimensions on the GGD car, but it sure looks good to me.
And it much better than Lionel's . . . . .
Os3r, if you go to this link and scroll down you can see the prototype. Maybe not as obvious as the line drawing, but the photo of the rear of the observation car certainly seems to show the roof higher at the rear of the dome than at the front.