It looks like the "pie cuts" were done to accommodate the shape of the se3lf-contained tail/stop lights that were installed. I'll wager the shape of the back of the light matches those cuts and bends pretty closely.

MIG?????? That's sort of like using a sledgehammer to swat a fly. That's a perfect place for some old fashioned oxy-acetyline welding or brazing.
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!