You'll be fine re-gluing the tread with Barge cement. I used to have to do that with the old Barums, too. That is one of the challenges of true, hand-made tubulars without a 'vulcanized' tread (i.e., adhered to the casing by hand-rolling with contact cement only, as opposed to the application of heat and pressure).
When you glue it, use a toothpick to spread the Barge evenly on both surfaces. Use more toothpicks to keep the two surfaces separate (this can be tricky, as the glue is really sticky). You need to make sure that the Barge cement cures properly before joining the tread and casing. It only takes about 30 minutes. After 30 minutes, clamp it gently, but firmly (there is still a tube in there). I use a spring-loaded plastic clamp from a set I got at Rona. In the old days, I used a textbook, that worked fine, too.