You are worrying too much, if you're going to go square taper there's a billion choices. All you have to do is look at the shimano docs and find out what the chainline on the double was supposed to be with a certain bottom bracket and simple math to figure out exactly what bb length you need.

Please read this http://www.sheldonbrown.com/chainline.html

You are unecessarily restricting yourself to a particular crank for no reason. The other thing about using converted cranks is that you'll usually end up having to put the chainring on the inside of the spider, aesthetcially unpleasing and widens your Q-Factor over a real track crank.