Are you talking about the engine compartment apron or the front frame rail on top. I know my does have a patch that was welded in by first PO and knowing the history of the car I suggest calling VA MUSTANG(540)896-2695. From where my car spent most of its life this was there primary part supplier and also try this http://www.oldominion.com/
If it's small, clean the area up, make a template of the hole with a piece of paper or cardboard, cuy out a peice of metal using the template and weld it in.
Or get a replacement apron, cut out what you need from it, and weld it in.