I did a conversion to square taper from cottered on my '72 Super Course. I tried a 5 series spindle, but it wasn't wide enough and the adjustable cup sank in too far....a 7 series 126mm spindle was the solution for me to get a Triple on there.

I bought it new on Amazon at the beginning of the summer.
here's one, although Its probably longer than you'd want. http://www.amazon.com/Bottom-Bracket...2258406&sr=8-1