Use a die grinder and a cone shaped stone, or a diamond dust burr. Ditto on elongating the frame hole- - - - -it's not as hard as the spring steel bumper bracket. Take your time and be patient- - - - -at die grinder spindle speeds, trying to force too fast a cut will make the bit chatter.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!