There are a few threads on Retrobike about welding disc tabs onto older frames. This may be of interest to those considering that option.
https://www.kickstarter.com/projects/46 ... t-fixtures
Re:
Dropouts and chainsuck plate mounts look correct. The seat tube is not flared at the bottom bracket the way Tange Prestige seat tubes usually are. Could it have been repaired(with a different tube) and repainted?
I think a vintage 450mm Syncros would be appropriate under the circumstances. You could either swing it or insert it(or both), to deter the would be thief.
Light tires for minimum rotating mass, a low gear, a stiff rear triangle, and knowing how to position your body during the climb are the main factors.
Having a light build is also critical. I have climbed some technical sections on mediocre bikes that others could not climb on high quality...