Again, I'm not the expert and probably have more questions than answers... So, are you going to butt weld at the red line? ....or what exactly?
Looking at your photo, I think I would cut along the red line a couple of inches at a time and peel away the excess, spot welding as I progressed. The other way would be to cut the entire patch out along the red line as it sits right now and use welding clamps or magnets to hold it in place as you weld.
That's the way I do them anyway. In fact, I'm doing a half dozen or so patches on my little Datsun right now. Good practice for me as I'm not the best welder.
I have found that for me, the most important time spent is getting the patches to fit properly.
