It's not. If your radiator cap was operating correctly and you had a 16 lb cap even if you had plain water in there it would not have boiled until about 260° F.

Good thinking. You figure you can get 6 qts of basic oil and some cheaper coolant for maybe 50 bucks and see how it goes. If it holds up for awhile, then you just change the oil and coolant to what you want to use and run it as normal. If you can get away with it, save yourself the trouble. You said the oil pressure is good and the compression isn't terrible. Worst case scenario you have to do the work anyway with a new head.

While your sister is using it, keep an eye out for maybe a parts jeep that you can get an engine and maybe other parts off of. I know around here parts jeeps usually go for less than 1000, and they usually can run and drive, at least onto a trailer.

Not saying all the coolant evaporated if that’s how you took that. What I’m saying is if a jeep was to have absolutely no coolant in it the gauge would read 210(this jeep was reading 210 and the cooling system was bone dry) so to the buyer who’s test driving a jeep for a short distance may assume the Jeeps cooling system is OK(by looking at the gauge and making sure it doesn’t overheat) even if there’s no coolant in it. That’s what I’m 99% certain happened in this situation.

Well now that I dropped the pan and pulled all the bearing caps and checked them I’m thinking that’s what I should have done. Bearings seemed good (my finger nail did not catch any scoring) and same with the crank journals. I’ll button it back up with a new oil pump, pan gasket and rear main. I even pulled the valve cover and did not see any visible cracks. But I would find it hard to believe it didn’t with how hot the engine got. The head studs on the manifold side melted the plastic retainers that hold the injector harness so it got pretty effin hot.

I had the impeller break on the water pump on my Wrangler.. 1000 miles from home on a road trip and my friend was driving - I woke up to my radiator being blown up and a massive overheating... a few times running the engine hot in the Maryland summer heat (it was 90-100 outside) and never cracked the 0331 head. The engine still runs great with over 300k on it.

That experience leads me to believe that it's the specific head - either it's the ticking time bomb (and doesn't require overheating to fail) or it's fine and no better or worse than any other head on these amazing engines.

That said - it's probably best to just go ahead and replace the head if you're willing to do it. I say this considering it's your sister and you probably want her to be safe and sound and not breaking down in a bad part of town.