Originally Posted by The Golden Boy

I guess I'd be inclined to say 43cm or longer.

Looking at the Vintage Trek site, there's a chart about the "tourability" of the 520. Since some years of the 520 were pretty firmly in "sport touring" territory, the chainstay length and the availability of cantis were good identifiers. 43cm was a good length- but I'm in the "longer is better" camp!