SLT-6 is 0.060 Over - Runs Hot...Why?????

A thermostat's rating basically sets the minimum temp of a fully warmed motor, not the max. If a motor has an overheating problem, a 160 degree thermostat won't keep the motor from warming up to 250-300 degrees.

I lean toward the radiator being clogged. Either from block internal rust getting into it as suggested above, or - what I've experienced - a radiator that sits very long with the coolant drained, but not flushed with fresh water, will clog up. I'm not sure how, and I was skeptical when someone told me this, but it happened to me twice - in both cases, the car would be fine around town and for short drives, but get out on the road, and after an hour the temp would start climbing, to 230 or more. In each case, boiling the radiator out solved the problem.