What's the hottest you would need to heat tires too for any given reason? Either at the track or at home rolling tires. Have some old thermostats from work, might try to use them to make a hot box.
If you are going to size tires in your hot box, consider that as well.
For prepping, 100* is more than enough in my opinion.
For sizing, I've heard of guys going to 300*! Personally, I would not want to heat my tires that much (especially with a ton of air already in them = bad things happen.)