Like a bomb went off under the hood...

Air.........these mopars are easy to bleed.......just fill the system with the intake heater fitting open, fill till you "hit water". If you don't have a heater, either put a valve there or a fitting you know you can remove, like a brass plug sealed with teflon, etc.

In addition to a bad new stat, clamp problems, or a stuck cap, if you underfilled it might be it overheated badly before the temp gauge reacted. This may have added to the extra steam pressure. Hope you didn't "hurt" anything.