Sorry I didn't get back to you sooner. 41" wide should be fine. We started to go wider be then the packing crate plywood was what determine our width. It turned out to be the best width anyway. Have you got any thing started?
Don, enjoyed reading about the "junkmobile." I want to build something similar. My donor garden tractor is an old 18 horse Murray. The outside wheel width of the rear end is already 41". It seems to me that I don't need to widen the rear end like you did; would you agree with that?