You can fit a 12" wide rim in there but with that you are limited to running a tire that is no wider than the rim. The guys running a 10" wheel and a 295-50 surely notice the bulge of the tire runs wider than the rim.
I run a 18x10 with a 295-45 tire. My tire is wider than the rim but there is room for more. The problem is not just with fitment at a stand-still but during movement, up-down and cornering.
See the pictures and notice the unpainted, sorta shiny spots? That is where the tire has made contact at the outer wheel tub. Chrysler, in some strange reason, made these tubs with a strange shape. In a perfect world, the outer wheel tub would have followed the quarter panel contour to allow the wheel and tire to sit out almost flush to the edge. New cars are like this, it reduces air drag and improves fuel efficiency. In old cars, it would allow much wider tires to be used.