I would do 3" at the minimum. 4.5 is better if you want to flex offroad.
remember that a "31 inch tire" is usually closer to 30", so a metric tire that measures 31.5 will be like a 32" tire
interior cages have been done. heck, you might be able to buy a kit these days.
dash swaps are a bit trickier. you might be blazing a new trail with that one.