I used an little air powered hack saw when I did mine. Remember to cut it shorter than on diagrams and fold the lip back in. Check out tricky's build for info on this. You actually dont see any of it once the arches are on. Saying that, if you are like me you will still want it to look good.
I heard that the old torana wheel arches just got a taste of the plasma cutter when they made the s5000 torana's ( is that the right model )... The one with the big arches that looks bad ass. Heard that a good way to see if they are real is look at the arches. If its all neat and pro looking its a fake

This is just what I heard. Not sure if its true or not. Interesting though.