I read through the posts... i see that you have tried replacing your thermostat, once. If it were me i would try pulling the Tstat out and see if that solves your problem. I have gotten thermostats before that are faulty from the get go. Might solve the over heating problem and might not, but you will just be out some wrench time. Just my 2cents

 

Just curious - would a cracked block as you described yield any other symptoms like coolant mixed with the oil (which I do not have)?

 

Just got a call from 'the shop' and the latest problem turned out to be a cracked head. My mechanic sent the head back to the machine shop and they confirmed the crack, and as luck would have it they have a set of good heads (from a bad block-engine) that they are offering up as conciliation. My truck is due back to me next week around Wednesday. I'm keeping my fingers crossed!