I bought a 88 Schwinn Sierra for the very same purpose. Mine is a 19" seat tube as well and should yield a more road bike fit and finish than a smaller and cramped mtb frame. The only things to look out for are the higher BB shell and the longer top tube. MTB frames varied from 80s to mid 90s. The long top tube is more of an issue as you get into the 90s. There used to be stems specific to adopting drop bars to mtbs...they are curved like a banana, with shallow extension but plenty of height. Don't see them much.