With my Twenty, I just ground the sides of the BB with a bench grinder, then used a file to dress them up flat and parallel- I had to take 1.5 mm from each side to make it 73 mm wide. The little chamfer on the edge shows about how much material to take off- it's just about 73 mm when the chamfer is gone.
Then I used a 73 mm wide UN54 BB that I ground all but the outer 3 rows of threads off. When you try to thread a 24 TPI BB into a 26 TPI shell, it will go in about 3 rows, and if you cut 24 TPI threads in a 26 TPI BB shell, you only end up with about 3 good rows of threads anyway. I'd rather modify the cartridge BB, which is expendable, than the BB shell's threads.
I just replaced the whole fork after cutting the headtube down a cm or two. I used this fork,
http://www.niagaracycle.com/product_...ducts_id=10801 which has cantilever brake mounts (the original brakes are anemic at best). Any old 1" threaded headset will do. Then, a quill extender if you want to get the handlebars higher.
160 mm cranks work fine. I have a Sturmey-Archer 8-speed rear hub with a drum brake in back.
Here's a picture:
