Kasskaboose, I noticed you asked your question about center beam flat dimensions but had received no replies. Before an answer could be given, someone would need a lot more information beyond simply "center beam flatcar dimensions".
Reporting marks would be an ideal start. Builder if known, would be a great help. Since the prototypes have gone through a period of evolution since the first cars built on regular 53 foot flatcars, in the early 1970s, your era modeled would also be helpful.
Because your posts indicate an interest in prototypical accuracy, I heartily suggest you acquire a copy of the Official Railway Equipment Register for the high end of your period of interest. For instance, if you model 1980 to 2000, get as close to a 2000 issue as you can. Ideally, you could also get a 1980 issue to "capture" the cars that were around in 1980 but didn't make it to 2000. Many modelers balk at spending the money on a publication that has no pictures, just miles and acres of numbers. Think of it this way. If the ORER helps you avoid purchasing ONE quality car that is incorrect for your era, it may very well have paid for itself. And, it is yours for life, to refer to, any time you need it. ORERs can be found at train and railroadiana shows (when they resume) as well as on-line sellers of such items. They also show up on Amazon and E-Bay.
Thanks for your help. I'll figure it out. No one answeredm so I can wait until the item arrives and make it from that!
I don't actually see a clear question here? There's no actual content in the first post, just a thread title?
Since I don't know what the question actually is...
Modern centre beam cars are standardized at 73'0" internal length between bulkheads.
Chris van der Heide
My Algoma Central Railway Modeling Blog
https://www.bnsf.com/ship-with-bnsf/ways-of-shipping/equipment/pdf/73Centerbeam.pdf
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.