OK: This might be helpful, or not. I had a 57 Jeep. I know not a bolt, but an interesting issue any way. Would idle for ever with out over heating. Would drive in town no overheating. But when you went over 45 MPH HOT!. Worked all the stuff that has been discussed here any way. Took the head off 2 times. Looked at every thing and still over heated when I went faster than in town. The real problem? Head was warped. Not much but just enough that exhaust gas was leaking into the water jacket. Only a little. So when the pressure went up, drove a little faster it super heated the cooling system. When I went just a bit faster it over pressured the coolant and pushed it out. Since I was on the road I could never tell why. Head gasket was not damaged, go figure. I would pull the head and check if it is true and flat and if it is cracked. Had the head milled and the problem went away. You might have something like this. Not a big leak, just enough to cause a lot of head scratching. Good luck and let us know what you find.
