Het Volk,

Aside from all th history, you said you have a clearance issue at the chainstays. You can increase the clearance between chainstays by dimpling in-between. While many here are totally DIY and will just take out a hammer and a big dowel and start beating away. I would send my bike to say, Gugie, and have it done with care and watchfulness.

650b has practical width up to 38 mm or 42 mm. To add a minimym of 3 mm per side, you need 48 mm free width. Depending on how you do fenders at that interface you may or may not need additional clearance.

In addition I'd add braze-on pivots, and I would probably set up for brazed center pulls rather than cantilevers of any sort.

I would not worry about crank length, but you can also test it and decide. Add a popsicle stick or a paper clip to your pedal, and test whether you are ever causing it to drag. This can set you up to make a good decision about BB height, pedal length, and crank length.