a town did this mod to a M813 which they put a 16yd dump bed onto .. They installed a fuller RTO13-dash-a-buncha-numbers (overdrive 13-speed roadranger) .. It was larger in all dimensions than your 7-speed of course, and it was quite a project to relocate everything to retain the live front axle, including having to move the engine forward which made them have to lose the fan .. Even-so, they still had to cut the shifter and weld a horizontal extender to get the shifter to come up through the tunnel in a workable place .. Because of the tranny they used, they never need to put it in low range anymore .. You did very good engineering and planning for your build, appears to be the best most-workable option without going through the trouble the town did