Look at where the rear sub frame joins up to the cab. The floor in that area rusts pretty quick.

also check under the front fenders up the top. There's that frame bit there that often gets rust in it.

If its got rust in replaceable panels its ok, but when its got rust in the chassis and everywhere else, your better off leaving it and going off to find a better one.

apparently the darker coloured datsuns recieved more primer when they left the factory, and so rust a lot slower.