Try a 26.6 or 26.8mm. Tange was the primary tubing source during this era. With the exception of Prestige, Pro and #4 , all their CrMo tubesets, including plain gauge, used a 26.8mm nominal inner diameter for the seat post.end of the seat tube. Allowing for typical clearance would result in a 26.6mm post but a 26.8mm post might be a better fit if the tube was reamed.