Y'know, for quite some time now I've thought about making a rear battery box out of fiberglass with separate rivet-on fenders (so it's shippable). And a front fiberglass box that would replace the entire cowl, side panels and front "grille" with a one-piece layup. One of the big advantages would be that there would be interior walls to contain the batteries and drain holes in the bottom so you could just hose down the batteries - no corrosion buildup in the crevices and corners. Do that on a steel tractor and you're just begging to rust some holes through your frame.
As for strength and frame rigidity, there should be no problem. The added thickness and one-piece construction would make that a non-issue.
Would anyone else be interested in fiberglass parts of professional quality that cost a bit more than steel? If there is, I might consider making molds and selling parts through Harold and Jim (Clean Power and Electric Tractor Store).