Sounds like a nice project,
Sounds like a nice project, you shouldn’t have any problem with the bottom bracket – as long as you can get it out and the threads aren’t damaged (and even then you could probably get them re-faced). So, as Cactuscat says you’re only real issue is going to be squeezing that nine speed block into a six speed space…
I must admit that I’d be torn as to whether to go the DIY route or take it to a framebuilder for that, maybe a good halfway house would be to take it to the bikeshop you’re going to buy the components from and see if they’ll do the stretching…
One other thing to bear in mind, did you want to swap over to Tiagra STI shifters? If you do, then you might have a problem with cable routing on a bike that was built for down tube shifters. On the other hand I think you can get 9-spd Tiagra downtube shifters. If you want to go for STI levers then you probably will need to visit a framebuilder to get some cable guides brazed on, if you’re doing that you might as well get him to do the stretching too. 🙂