I am sure there were custom frames in the 50-60s.... but I wouldn't have any idea where to have found them. I am not a historian... but I do believe the bicycles sizing/dimensions... were more predictable (top tube to seat tube) back then. Concept of size, fit, and even shape of bicycles have changed a lot (for bicycles) in the last half century.

First... I am not qualified proclaim either vintage or modern bicycles to be of a better fit.

I ride mostly modern bikes.... but do enjoy keeping vintage and classic bikes as well. I did in my searching for a vintage bike buy and restore an old Schwinn Continental. Same size as (and so much like) my old Varsity... (only it was the color of an old Schwinn Corvette I had owned). The Continental fit and rode exactly as I remembered the Varsity. Narrow handlebars and all.

I re-sold it almost immediately after restoring it.