It is really best to cut the patch to fit the body, not the other way around. Its kind of like trimming your sideburns, if you mess up you have to cut higher and you run out.
I would try to make the seam someplace where it either wont be noticed or looks like it was done that way at the factory. On the firewall, the spot where the bend is for the floorboards seems like a logical place. The higher up it is, the more likely you will see it.
You will have a hard time duplicating factory spot welds, and you will definately warp the body out of alignment if you remove the whole firewall unless you take very careful measures to brace it. This is major surgery.
