First thought: Can't wait to see pictures.

Second though: All of my 126mm bikes have remained so. 7-speed cassette hubs are "obsolete" but plentiful and long-lasting, so I build up new wheels around them. 7-speed cassettes and appropriate chains are also inexpensive and widely available. Don't feel that you need to stretch your bike to 130mm (or beyond) in order to take advantage of modern cassette hubs, although it's not "wrong" to do either.

Last thought: 650B is wonderful, but conversions are often a "project" involving measurements, tests, frustration, and compromise.