You'll need to replace the spindle.
IF you stick to sq. taper, you could probably reuse your existing cups and avoid the expense/hassle of finding a replacement BB. (is it French or....? problems)
Loose screws sells a spindle that could probably work. It's "offset", so if you used the "short" end on the DS side, it would probably work, although chainline might not be ideal....Probably too far out.
http://www.loosescrews.com/index.cgi...d=583333426343
I might be doing a similar swap in the future, making my brothers early 70's Asian 10 speed into a 15 speed.
Else, things get more expensive.