I added braze-on water bottles mounts, cable stops and rack mounts onto my 1973 Schwinn frame. It was not too hard after the first misstep.
I used MAP gas, 56% silver solder, and generous flux (which came with the solder).
So, for those of you who may have wanted to but shied away, go for it. It's not too hard.
Is the first page, of 4 pages so far on this project.