i rolled the arches on my wagon (as you guys over there would call it )

the rears do not really flare out like the fronts can be made to do, as you say the structure is rigid around there. cutting it and welding in a section would allow you to go wider. also, as you get near the ends of the arch, it gets very difficult to roll them as it gets stronger



good luck with it, and let us know how you get on