I think I would straighten it up a bit more and put filler back in it. When applied correctly the filler stays put. Apparently the old filler was in there pretty good. Filler is not a bad thing. Its bad when covering up rust or gaping holes. I don't thinks its worth all the work cutting the car apart. Cut it apart or put filler in it, its going to look the same when done. Filler is bad when you do a bad job with it and end up with a wavy panel. In my mind filler was meant for this exact type of issue. I'm sure others will chime in with an opinion. If you are handy with a welder then putting a new panel in might be easier.
I was thinking the same thing. A good body/fender guy can do that and look perfect, last forever since there would be no moisture or corrosion on the back side. Fixing that corner to original is going to take some serious work.
If your still wanting to graft in the corner contact Dan's Garage in Kennewick WA. I know he has several 70-73's that would probably work. His web address is DansGarage.net. Since covid he has not had regular hours and rarely answers his phone. I would email him at [email protected] he should be able to help.
Before you do anything, can you make a poster board template of the passenger side that goes from the bumper mounting holes all the way to the side marker light? It looks like that whole are might still be pushed in quite a bit. If so, you'll probably need a porta power or bottle jack to push the corner back out and then you could work on cleaning up the dents and curves.
Making templates on a good side helps a lot to fix a bad side. I did this to fix an unobtainable 65 el camino quarter panel.