If you do weld it, it might be OK. I don't think you need to worry about the brazing in the joints since heating the lug will only draw the braze further into the joint. I'm not a frame builder, but I wouldn't worry too much. Make sure to leave the bottom bracket in if you do decide to do it so that the shell doesn't end up warped and stuff. I have a 52 cm schwinn world sport and a peugeot U-08 that could be yours cheap.
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