Brian that looks BAD
but i think the diamond plate would work well but get the thinner stuff like 14 gauge maybe, 1/8th inch would be a little hard to bend. but cut out the bad and over lap putting the new under the old. this way it supports the old and if you hit hard it doesnt simply rip the body mount up into your leg. trace it out onto some old cereal box then transfer that to the steel.
Can you pull the cab?? or unbolt all six mounts and run some 2x4 or 4x4 under the cab over the frame maybe.
