Like Dave said, change the Thermostat, you're in Colorado, altitude changes the boiling point, also reverse flush the block and heater core, pull the stat out, hook a garden hose to the top hose, pull the lower hose and back flow it. Go back and forth, straight flow, back flow several times to clean the block. Also check the radiator out and make sure crud from the block isn't in it if the original was as bad as you say.
Probably wouldn't hurt to change the water pump and check behind it, the divider plate might also be bad and the bypass might be clogged. All in all about a days work to do all this, but you'll know it's as clean as possible without a tear down.
Up to you if you want to try and pull the block drains, iffy getting them out without breaking on an old engine, but reverse flush should get most of it out.
Wanting a Custom fit in an off the rack world.
I don't have time for a job, I just need the money.
