I have found that it is usually not possible to build braze on
shifters onto a band. I think (for some reason) the bolts
for braze on shifters are different lengths to clamp on bolts. Also, the washers seem to be a different size too.
I tried it the other way round to you (building shifters from
a clamp onto a braze on) and it didn't work at all.
This site looks to have a couple of sets of band on shimano shifters, but not sure if they are exactly what you need:
http://www.bike-vintage.com/27-downtube ... ifters?p=2