[MENTION=488254]agent9[/MENTION] - Check to make sure it is the frame and not the bolt in the RD hanging up! You can't put back what you remove with a file!
Your frame is built for "Nutted" brakes. I always go with what the frame was designed for and don't modify the frame to accommodate upgrades, your choice. Keep in mind that once you increase the hole size, the tube material thickness may not be enough to hold the brake. It is curved unless it is a brake block. The older frames used a tube and not a solid or thick walled bridge. The nutted brakes had a curved washer to mate to the curvature of the tube. You should use a reverse curved surface for the inside of the tube if you drill it out.


LC_StTb_00, on Flickr


P1000503, on Flickr