Violator has a point though, if you seam seal it you can eliminate th moisture problem. Go one better and coat the inside of the lip with some POR, roll, and seam seal. You’ll keep the structural integrity and not have any miscuts. Cutting it will be tricky as maintaining a perfect line with a cutting wheel on a radiused edge can be tricky and then you will still have to deal with the exposed paint and surface rust. Plus, if that cutter slips…
If you’re set on cutting it, I’d use a 2" cutting wheel on a good air pistol-grip sander, then just block it down and coat the cut edge.