The Classic Cadillac Forum Discussion, Lower Door Panels for 76 Eldorado Conv. in Past Cadillac Vehicle Discussion; Does anyone have the lower door panels for both doors? Or are there any parts houses that carry these?...
Check this stuff. http://www.rustrepair.com/DOOR-BOTTOMS-OUTER-LIP.HTM
they're 58" long and have been bent along one edge, the idea is to cut the repair area, and fit these in as a patch, and use a hammer and dolly to finish the bend to conform to the door frame.
I purchased one of these for my Eldorado, but for use in another area (trunklid) they're 20 gauge steel, and cheap.. available from your local bodyshop supply shop.