Front derailleur clamp size

I've an old (1999) Lemond Zurich frame that I'm restomodding. I'll be putting Ultegra R8000 on the thing. The front derailleur is available for a braze-on, or as a clamp. The frame does not have a braze-on tab, The size of the two clamp sizes listed for Shimano FDs are 21.6-31.8mm, or 34.9mm. ...