Overheating issue with the old slant 6

OK on the pressure. Yes, it might be that hot, but that is kinda hard to exactly say; it's too subjective and variable to be any sort of good test.

See if you can borrow a cooling system pressure tester that will fit on the radiator cap location. You can pump up the system pressure with this tester and see if the pressure holds and does not leak down; if not, then you have a leak somewhere.

Also, have you looked at the spark plugs to see if there is a fine, whitish granular deposit on any of the plug tips? That is a sign of coolant entering the combustion chamber.

Continuous bubbles out of the radiator is a common sign too. If a blown head gasket is the problem it sounds like it is pretty far gone with the apparent speed of the pressure and heat buildups.

I asked for a pix of the cap to just be sure it is a pressure type and of the right pressure rating. How did the fluid separate out? Rust (bottom sediment) or oily at the top?

I would be:
1) check the coolant coloration casue and the plugs as suggested
2) looking at the coolant in the top tank of the radiator while starting up from a cold start to see if the bubbles keep coming (sounds like the thermostat opens OK)
3) trying to verify the actual temp other than using the temp guage in the car
4) if it is really that hot, try to borrow the pressure guage; a few autoparts stores have them