Re: 550-1 Mounting a scope
Find another gunsmith, tens of thousands of model 550/550-1's have been drilled and tapped for a scope, even the factory did it if requested. Weaver makes side mounts that fit these quite well, look for a Weaver N3.
As to your other question, if you haven't figured it out already. Sounds like the threads may be stripped on the plug. Just loosen the screw on the receiver plug retainer (little "door" on the back of the receiver), and then unscrew the plug all the way, if it's already stripped your not going to harm it.
When re installing it's important to align the threads correctly.