I would put a 5mm spacer on the outside of each dropout to fill any gap in the bob's yoke. That should work fine, will be the easiest solution and will prevent you from having to respace your vintage frame.
Respacing a steel frame is easy enough, but afterwards you will have to either:
1) replace your rear hub (with a non vintage part, if that matters to you- it might matter to me, depending on the year of the stumpy),
2) add spacers to the existing hub- which will weaken the rear wheel by increasing the chances of bent or broken axles (longer torque-arm between the bearing races and the dropout).
Even if you want to do option 1 and upgrade from vintage to modern hub, you will have to respace the frame from 120 - 135mm. This is a large spread, and it means you will certainly have to re-align the dropout faces, as they will no longer be paralell- so plan to find an LBS or framebuilder with the proper park tool to do this. The second problem that might occur with such a big respacing is that the brazing or welding of the brake bridge or chainstay bridge can pop, which is fixable (by a framebuilder or experienced metalworker), but a royal pain. If you do respace the frame, use fiberglass strapping tape to take the stress off these two points of the rear triangle before doing the levering.
hope this helps