Id get on the Internet and find some truck shops near you that do Bodies and get quotes, Having the rear of the box, doors, frame removed isnt really that hard its all Bucked and Riveted with usually some corners welds, If its a good box and structurally sound, A new box is gonna be 5000/10000 bucks....Morgan, Supreme, and other name brand builders modify and build/customize it how you want it....I used to work for Trailmobile/Fruehauf back in the day building Reefers and Vans.
Im sure there are Truck Body companys around your area, than can make it how you want it, you might look around some of the truck/trailer junk yards and find a Semi trailer frame, doors, Icc all together you could buy and have refurbished and mounted, new Door rubber, etc...
Your going to want to measure your box, and frame etc to be sure you can mount a larger door, but typically we used to do that kind of thing often, its only metal and a good box builder can customize it...we used to make custom doors from Blanks, etc all the time...