Regarding the bottom bracket shell, do framebuilders modify it at all or just weld the tubes to the shell as they buy it? I know that there are a lot of different types of bottom brackets so how does that fit into building the bike and selecting a bottom bracket shell? If I build a frame, do I need to pick the exact bottom bracket before I start?
What's the best way to braze seat/chain stays to track dropouts? Do you need to notch the stays and then braze it in and fill the gaps?
you don't have much choice in BB shells. Generally, there are a couple of degrees of slop available, and after that you might have to bend the sockets. The lower head lug seems like the one I have to modify most often.