'72 superbe,

When I was building up my Twenty, I actually went down the list of crank/BB upgrades that Sheldon listed and eventually ended up rethreading. It is near impossible to source a long enough spindle...I even contacted Harris Cyclery. They sent me their longest one and it was not long enough. I tried the un72, but as you've found out, they are no longer made and it is difficult to find one in the proper length. My rethreaded BB works great, and even better, it was a cheap fix. Save yourself a headache and either rethread or source a *** bb,

Juan