Re: How do you extend a bike frame?
that frame looks like it has an oval (or close to it) downtube. that wouldn't be easy to find. probably have to find a specialty metal supply place and order it. which wouldn't be cheap.
pretty much all bikes were brazed in the olden days (pre 40's) and the practice just carried on with some bikes. it's plenty strong, cheap, and makes those cool looking yellow/gold welds...