From that picture, I'm pretty sure of one thing. Wrong bike shop. Since you went to them to have a rear derailleur mounted, I'm guessing you wouldn't be comfortable trying to re-align your frame after reading about it. I suggest you find another LBS or co-op or local C&V crowd to help you out. If you enjoy older bikes this won't be the last time something comes up. I would not be inclined to go back to the first shop for any more help or "advice".

Nice looking bike!