Hi VR4, I think it looks like the Schwinn Letours era 1974,,,,,1976.by headlugs. I commuted on a steel frame and wheels for an extended period. If it has a 68MM bb shell width you can put a newer crankset on it. The rear dropouts were 120mm 0r 126 MM.
Measure the frame. The seat tube and the top tube. I was pro fitted in 1980 for 23" frame. When I finally got a 60CM frame and put my level on the saddle and measured it,,like from the nose of the saddle to where the handlebars. I would put my elbow on the saddle nose and have your fingers touch the stem. I've always seen catalog pictures of bikes. Seems they always show the saddle centered on the seatpost from a side view and its level. I also seem to look centered over the bottom bracket. If you draw a plumb line from bb spindle.