I just got a new catalog last week that gives the breakdown of the vin #'s, it actually said that a coulple yrs (71 and 72 were a couple of them I think) vin #'s that acutually will not tell you if its a true SS. I was suprised. I will bring it w/ me to work tomorrow since we still have no power at home or phone line.
The book also gives the break down of all the numbers for the orig. color, int. color, etc..

As far as rust, check at the base of the back window under the two bottom corners, thats real common, where the seam is for the quarters and the piece under the window. Also all around the trunk lid, look inside the trunk REAL good. Run your hands in the trunk down in by the wheel well. If it ever leaked it will be rusted down there too, but you have to reach way down there and all around to feel it. Check from outside by the tire all along the wheel well from the outside also and the lip of the wheel well. If it has rocker panels, look real close at them it could be rusted under them too. Also around the front window pillars. I can name so many places its not even funny, can you tell where I've had rust . But I still wouldn't sell it!

What kind of shape is the car in? Is it somewhat restored? Power brakes, and steering? Drum brakes all around?

Check all the rubber around the windows, trunk, doors etc., also, thats potential for a leak. See if there dry rotted.

Ask if he has reciepts from what hes done to the car. Just some things down the road from buying mine I had to do was a steering box, all bushings, shocks, springs, misc. brake lines from being old and rotted. It's just what goes along w/ old cars
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Good luck!