Here is a suggestion: Go to a local shop that specialises in rebuilds, like Open Bicycle in Union Sq in Somerville or Superb in Boston. They very well might have a good vintage frame in your size and even if not, can give you some helpful advice.
Otherwise, there are so many bikes on our local C-List that would work well as conversions, it is hard to choose. I suggest a late 70s- early 80s Peugeot or Motobecane in your size; those seem to work nicely.