I find this stuff interesting. On the History Channel website it said the first employees started working at their offices in the Pentagon in April after the September start.
The 700,000 tons of sand for the concrete was dredged from the Potomac! No way could that be done today with environmental regulations, screening for toxins in the sand, etc. (Hmm - probably a good way to clean contaminants from a riverbed - turn it into concrete.)
That's pretty much what our guide said. The reason they could build the Pentagon so quickly was because there were no environmental regulations.