If the inner fender is rotten as well - common around the battery tray - why not consider replacing that whole frame rail and inner fender with a good used one.

Cut the spot welds out down the rad support
Cut the spot welds out at the firewall bracket
Cut the spot weld out on the cowl bracket
Cut the spot welds out along the floor
Cut the spot welds out at the crossmember
Using all those holes, align and weld the new section in