Tom,
Aaron is right. In my opinion, there are two types of
older Airstreams (especially 70s)..... those with rear floor separation issues and those whose issues have been addressed.
It is a real job, even if you know what you're doing (i.e. - done it before).
After hearing what L82Stingray paid for his, I was tempted to leave the pizza business (but then I recounted the hours involved on mine and thought better of it).
I personally would not purchase a used Airstream without drilling out a few rivets and inspecting underneath the bellypan just to see for myself. (Bring some replacement rivets so the seller doesn't get too upset.)
Or....don't pay more than $2-3K and you can afford to put 300+ hours into it!
Good luck!