I checked every mustang vendor I can think of and while I can find full door skins for a 69-70, I cannot seem to find anyone who sells just the lower skins for a 69-70. Does this mean that such a part is nothing more than a Questing Beast?
If you are referring to the lower 5-6 inches of the door, then the regular door panels will not cover it. You will need to look for deluxe interior door panels to get coverage in that area, or have custom skins made.
Mustangsplus has door skins for 69's. $139.95 a piece. Right side is part #10600 and left side is part #10580. Give them a call. 1-800-999-4289 There is no such thing as a lower skin. You either make you own piece for the small damage spots, or replace the whole thing.
If you just need the skin, what about having a sheetmetal company take the same guage steel and have them fold over an edge the amount that the stock skin is. Have them do 2 for you, one for each side. How far do you need to go up? You can have them step the top edge back the thickness of the sheet metal and this would give you a lip to weld the original skin to. You'd have to work the ends by hand though.
There USED to be a lower patch panel avail for the front and rear lower outer door (wow! what a discription ). I dont' think they made the entire lower piece available (like for the early models). '69's and 70's tended to rot out at the corners not in the middle.