Some time ago, there was a thread about using adjustable stems to
determine what stem length would be exactly right for YOU. Doing my
usual casual morning troll on the local Craigslist, I found the
following (off topic) bike:
http://washingtondc.craigslist.org/
It is expensive, but so is the frustration of the alternatives (I still can't rationalize buying one for myself at $200 + adapter to quill-type steerer). Of course, the only reason to note it here is to help you find the right fit for your vintage bike. At that point, you will certainly not deface a proper on-topic bike with such an ugly artifice, no matter how convenient you find front-loading stems. :-)
harvey sachs
mcLean va