[Jimmy P]

I'm assuming the patch panels are for replacing the entire lower fenders?
I've always found that using as much of the original fender's steel yields the best results and saves a lot of work and time.
How much good metal do you have left in the originals? If you just have a hole in the center, I would just cut out the rot, make a patch and weld it in. The new pieces aren't shaped the same as N.O.S.

The best and most professional method would be hammer welding the new patches. It takes talent and practice to pull it off.
Mig welding a piece in isn't all that hard. Make it as flush as possible, not creating any high spots. Do a stitch weld alternating the heat spots. Keep the metal cool so it dosen't warp.