the car body is layers of steel that are connected usually by the spot welds. To repair, and doing said repair by making your own panels, yes I also suggest smaller segments removed at a time. I also suggest full perimeter welds of the replaced parts. The body on your car is a bit more gone than many in the area of the kick panel, floor and inner rocker and below the quarter light is typical of this year vehicle also. I salvage a 51 Plymouth Suburban that did look very much like your damage. The task is not all that hard but some of the standing on your head to weld will qualify you for the Olympic gymnastics team.
I could send you some pic but would not post here....close to maxing out my photo allocation. IF interested, send me a PM with your e-mail....and as stated...working one segment at a time. I however recommend the front at the cowl to begin as so much is going on here that ties the body together. Making your panels will not be hard with bead roller....but you may wish to have another set of hands to turn the crank as you guide the metal. It can get involved. Outer rockers should be readily available C2C should be able to help. DO NOT EXPECT perfect fits even with shaped panels...it has been 30+ years since I have seen factory replacement rockers. The rear pan under the quarter lights will likely continue into the wheel opening with your repairs...again, layers fixing the deepest layer first cutting a tad beyond the next layer so you will have good metal to weld the next layer in place.