1st hand info on Mt Rushmore room:

The room is only a rough hewn opening deep enough to walk into (roughly 20-30 feet) but not much else. It still has rubble from drilling. Supposedly it was to become a “Hall of Records” but the funds were cut off during wartime (not from diverting funds like I thought). There’s some good photos here:

https://www.nps.gov/moru/learn/historyculture/hall-of-records.htm