1-3 mm isn't much of a margin for error, with hitting bumps on the highway. How much clearance is there for the brake calipers, both front and back?

The tires/wheels are from my 85 Cherokee, 15 X 7 with 4" back space, 30 x 9.5 tires.

I had a close look at my 15" wheels in action.
Just for grins I had my wife push and pull on the top of the tire to see just how much a steel rim would flex. I was amazed that my small wife was able to get the rim to flex that much... This bolt closed 1/2 the distance to the rim with the little bit she was able to push and pull on it. If I were to be going fast around a tight corner, the combination of the weight of the Nitro and the side force of going around the corner could make that bolt hit the rim...

Now some Pics of the caliper clearance. Less than 1/4" at the closest point.




IT'S NOT WORTH THE RISK!

These are coming off as soon as I can get them off!

15x8s with 32x11.5 tires

Personally I would not trust the combo.
Here is a test I did a while back. 2.5" lift and even the small size tires they rubbed.



Now you may be able to make a 15"rim work if you ran less backspace. But then you would have to cut the heck out of things to be able to turn. You could also go up higher then a 2.5" lift to give you a little more room while driving normal roads. But you can forget about stuffing a tire during flex.

ScarO's post was spot on and what many do not want to hear, with little room in there any thing can cause an issue.
As I've stated before used to manage tire shops and always had " my brothers cousins inlaw's grandfather ran that size "
finally gave up one day on one customer and said fine you want those wheels I'll sell them to you but they will not work
Guy bought them took them home, installed them then cam in to show me I was a dumba**
2 weeks later he installed new brake pads on the front, guess what the thickness of the new pads pushed the caliper out far enough that the wheel wouldn't turn.
He bought the wheels I recommended in the first place

I currently have a 4" (well, closer to a 4.25"). I have the pinch welds pounded flat. And I only plan on 31×10.50s because you can find a lot more tires are available.

Can you take a pic of the clearance near that lower bolt? What's the backspacing on those wheels?

I currently have a 4" (well, closer to a 4.25"). I have the pinch welds pounded flat. And I only plan on 31×10.50s because you can find a lot more tires are available.

Min pinch welds are pounded flat also no problems with my 32s and my 4.5 lift

As long as you keep that sucker on the roads you will have no problems with 32's. As soon as you try any actual wheeling whatsoever, you will rip off your fender flares with that combo.

That was my big question. 31 x 10.5 and 245's are almost exactly the same size. Is the back spacing that much different between the 15 and 16 inch steel rims?