58 was a down year for the auto industry. However Chevy sold LOTS of the somewhat different Impalas. Later, in the 60"s, there were several kids that drove 58 Impalas at our high school. One was a convertible that I really admired, and another kid had what was supposed to be a 409 in his hardtop.
One of my friends had a 58 Bel-Air hardtop. It was similar in some ways to the Impala, but was somewhat smaller and lighter. The roofline was quite a bit different than the one on the Impala. As I remember it, that car was really quick with a later 327 and floor shifted 3 speed in it.
58 was the first year for the frame/chassis that GM used for the next few years. It was quite a bit different than the earlier chassis, and I always thought the earlier cars handled better.
I don"t think that 58 Chevys would be considered rare, but I would have to agree that I have not seen many in recent years. Especially compared to the 55 thru 57"s--I see them on the street often. Maybe the 58"s rusted worse, or maybe the tri 5"s were just valued more, so more survived.
It would be interesting to find out what happened to those neat cars that were at my high school and presumably other high schools way back when. I know a couple of them still exist, because I have talked to their owners, who kept them. I wish I had kept my 64 Dart convertible and 48 Plymouth!