If you are having problems threading in the cup, my recommendation is to take the frame to a good LBS that has the proper tools (look for an older LBS with an old-school feel) and have them "chase" the threads with the proper thread tap. This would take less than 5min to do and probably cost you about $5 to $10...if you bought a couple things like a water bottle and a tube they might not even charge you for the job.

Or...if you were able to partially hand screw the cup in a bunch threads before it started to bind, and you believe the binding is due to slivers at the bottom bracket-chainstay junction then you can slide a tool on there and give it a tad bit of elbow grease...tighten it slightly then back it off, tighten a bit further then back off again...repeat until the parts thread smoothly....then pull it out and clean it out with a wire brush and reinstall to proper torque.

Though if it were me, I would take it to a shop to have it chased. Much less hassle than potentially ruining your frame.

-j