That's one of those projects I wished I had paid someone to do. It looks good but took me about 10 hours total (didnt include grinding welds smooth). Fit and cut, repeat, repeat, repeat etc.
I butt welded mine below the bodyline and 2 inches from the side scoop. Also was a good time to clean out and use rust converter behind all those areas you can't usually get to.
I started by taping off the areas I was replacing. Then made additional marks with height measurements, not sure if was really needed but wasn't going to move the car until I was done, just didnt want to chance the body sagging on me. After that I chopped out along my marks and used the cutout as the template for the new panel. I fit and trimmed for a good solid hour just to get it perfect. After the final fit, I tacked the top in about 10 places and rechecked all my gaps and alignment. After that was some good quality (sarcasm) time tacking around the whole thing with plenty of spaces in between the welds, you want to be careful not to build up the heat too much and warp it.