Seeking specific dimensions of a grand piano

The question is interesting, but I am at least a little bit interested in why you want these dimensions, i.e. is it so that you an build a cabinet ?.

When considering such a project myself I quickly realized that existing pianos are to some extent constrained by the rest of the piano.
"kEYBOARD" cabinets need not be constrained by any need to accommodate hammers and strings.

Also, any music stand`could be at a height appropriate for YOUR individual needs - or could be replaced by a monitor to display digitally stored scores.

 
I  wouldn't limit myself to the furniture designs of 100 years ago

Sorry for taking longer to reply than I had planned, but I needed a little extra time to check me library for these details since I don't have a grand at my disposal (and coronavirus and an earthquake haven't made things easier either).

Unfortunately, most of the information you're after is usually considered too trivial to print in diagrams, spec sheets, or patents, and varies significantly from manufacturer to manufacturer and from piano model to piano model.  Part of the reason it's treated as trivial is that most rebuilders rarely have a reason to replace that part of the case, and if they do, the case is usually there to be measured.  I would recommend calling a piano rebuilder or a good registered piano technician and asking for the measurements in mm of each part you're curious about.  Since you're not concerned about the make and model--or at least not very concerned--whatever results you get should be representative of most instruments in the industry.

For the technical jargon, it appears you want to know the height of the fallboard, the length and height of cheek, the height of the rim, the height of the music shelf cavity, the height of the keyslip, and the height of the keybed.  Some of these can be found here: https://www.steinway.com/pianos/steinway/grand/model-d

Key top height is usually standard at 730mm, which is generally a lot more important than the thickness of the keybed or keyslip, because--as long as the area below the keys isn't unreasonably tall--it only matters that you are able to adjust your bench to fit that common key top height.  Most manufacturers (like the links below) will specify everything in relation to the key top height to keep measurements references consistent between pianos, because technicians have enough to cope with before adding different measurement schemes and baselines for each piano maker.

https://i.pinimg.com/originals/72/fe/19...6affa7.gif
https://www.piano.christophersmit.com/keyblock.html
https://i.pinimg.com/236x/9e/20/90/9e20...design.jpg

It's possible that there's more information in "The Official Guide to Steinway Pianos" by David R. Kirkland and Roy F. Kehl, but I don't have a copy right now.  Though a few forum members do.  I also don't have access to any instruments but my own during the "social distancing" time we live in, but I wish I could help solve your need in one post.

I have checked "Piano" by John-Paul Williams and "Steinway" by Ronald Ratcliff, and I didn't find anything that would help.  I didn't check "Piano: The Making of a Steinway Concert Grand" by James Barron, "Piano Servicing, Tuning, and Rebuilding: For the Professional, the Student, and the Hobbyist" by Arthur A. Reblitz, "Pianos Inside Out: A Comprehensive Guide to Piano Tuning, Repairing, and Rebuilding" by Mario Igrec, or "88 Keys: The Making of a Steinway Piano" by Miles Chapin since I don't have copies of any of those, and I don't if anyone in the forums does (and to be truthful some of these books in the list are the finest resources on the market and some are almost worthless).  While I doubt any written source will have the information you're looking for in a single diagram, these books would be the most likely to have it if any books do.

I did a quick look at Dolge's "Pianos and their Makers" which is a wonderful resource but didn't have anything that I could see that would be helpful to you, but being a free, out-of-copyright book, you can take a look for anything I may have missed here: https://archive.org/details/pianostheir...mode/thumb

As I said before, I would recommend contacting a rebuilder or registered piano technician in your area.  Explain your needs and project--be honest and direct--and see if someone helps you.  You may need to be patient, but somebody will have what you're looking for or be able to take the time (which I would gladly do but I can't because of quarantine plus road cloasures after an earthquake--so I wouldn't recommend that you wait on me).   Sometimes rebuilders and--more often--technicians have uprights and grands that will be sent to the trash, and you get them for free or under $200.  Since you don't need anything but the case to be working, one of those doomed pianos could be gutted and retrofitted for your needs, so having local contacts for your project could help you even more then just with dimensions.  I would say the average in my area would be about 10-20 pianos a week are sent to the trash because they're not worth the cost of rebuilding, and it's usually a rebuilder or technician who makes that call.

If you put labels like A, B, C on your brackets it would be easier to answer.