Wow, you are good. That is pretty much exactly what we did. We used a hose clamp to make the arc and masked the entire thing. Even still, it was rough and filing can only do some much without damage. Since you need to remove the welds on the back I chose to have a shop do the cleanup, removal, and respray. I use a very high end paint shop (read also, very expensive but a place I trust) and they charged me 150 bucks for everything. My panel had previous damage so that included fixing it correctly.
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so far i haven't had any chipping doing the cutting process. I will post pics soon as i get it back probably tomorrow

 

If you try to cut your header panel with out repainting the panel you wiil enjoy a long lasting relationship with the bitch that never sleeps. Have a nice rusty future!

 

Even though i didn't chip it i am going to repaint the whole car and panel so no biggie