Originally, neither Smallville nor Metropolis were ever located in real-world states. They were just a generic Small Town USA and Big City USA.
I believe that it was one of the script drafts for "Superman- The Movie" that placed Smallville in Kansas. However, I don't believe that "Kansas" was ever actually mentioned in the final movie itself. I think the public was first exposed to the "Kansas" location in Maggin's novel "Last Son of Krypton" (perhaps he got it from reading an early movie draft) but was then pretty much ignored until Byrne's reboot, when Smallville became firmly and perpetually lodged in Kansas.
The site of Metropolis seems to move depending on the writer and/or editor. In Siegel's mind, it was a thinly veiled Cleveland. In the movie, it was New York. I think that at some point in the DCU, it was located in New Jersey (or maybe that was Gotham). It's probably been located on the west coast, the east coast, and everywhere in-between.
My old brain gets foggy these days, please correct me where I've slipped up.