Congrats on your "new" 66 Overlander! We owned a 66 Overlander - bought it in 1980, towed it with a 75 Ford Custom 500 with a 400 engine - sold it when our nanny totaled the car in 85 . . . .always regretted selling that trailer . . .
Anyway, to your hole in the floor - I don't know what the couch in your rig looked like, or if it was original - we had rectangular heater ducts running under our floor, with "exit" vents in the actual floor. Our kids kept dropping their Leggo down the vents, so, in the summer, we covered over the vents.
As for the table, it may have been modified. However, since the Airstreams of this era were often semi-customized, what you see in the manual may not be what was produced.
Check the wiring used as well. During 'Nam, copper was deemed for military usage and your unit may have aluminum wiring. If so, you might want to replace it with current "code."
You'll probably have some other "suggestions" from forum members - good luck in your project!