The making nut plates thing I understand, but you're not going to be able to weld the steel nut plates to the aluminum tub...
Anyone can make the plates you need pretty EZ if you just go to a sheet metal shop, ornamental iron place, or maybe you even know a muffler place you don't mind asking. Buy the metal strips at HD/Lowe's, cut, drill, get the nuts you want, and then just ask someone to tack them in place. Paint them after they're welded up.
WRT to attaching it to the truck, if you must, just rivet them.