If this isn't a restoration, but merely a keep-it-going repair, then you
could add the stiffening as separate pieces welded to the bottom of the
patch. I would take a piece of your sheet, about 1 1/2" wide & as long
as the needed rib, clamp it between 2 angle irons in a vise and hammer
it over. Tack weld in place.

Or take a piece of thin wall conduit, slice it in half (steady hand on a
bandsaw should do it) and tack weld in place.

Bob