Here's a couple of thoughts.

I just got through making a scoop and the opening in the hood on my 57 Plymouth. I used an air powered cut off tool. It worked really well without warping the hood and although the cars is going to be repainted, going slow it didn't even really damage the paint next to the cut. If I had actually cared about the paint I would have run masking tape around the outer edge I was going to cut.

To finish off the opening, I used a die gringer with a round stone to curve the corners instead of leaving them at right angels (you can't cut curve with a cut off wheel).

I then took a fine file to to the cut edges to smooth them a little. To finish off the opening, I bought a roll of door edge guard
to slip over the edge of the opening.