I think the first place I would start, at this point, would be by popping out one of the rear most freeze plugs and seeing how much gunk is in the block. That might solve the whole issue.
I've seen blocks so full of gunk I pulled a freeze plug and it was packed full of sediment, there wasn't even any sign of coolant having been there for a long time. That one even flushed clear before the freeze plug removal!
If that isn't the problem, you need to look for something strange. It well may be a combination of little things adding up. You have swapped a lot of parts. Now its time to shift or relocate some of those parts and see what happens.
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I had the same problem, turned out the fan was not spinning fast enough at highway speeds. Was fine at idle or low speed all day..
I've experienced this one before as well. A simple test is to pull the fan off and hit the interstate and see if it gets hot.
Is your temp gauge mechanical or electric? I've seen electrical gauges miss read when the charging system is in a high rate of charge. They work fine at lower rpm, but as the voltage increases with rpm, things go array. Like stated earlier, if its really overheating, there should be signs of it doing so, more then a gauge reading.
While your looking at your pulleys, check to be sure the groves are not worn. I've seen recently (these parts are all getting pretty old now) where the sides of the pulleys were worn off and the belts were actually running on the bottom of the pulley instead of on the sides like they were designed to run, The result was slipping belts as the engine rpm increased. At lower speeds everything was great.
The sides of the pulleys need to be "V" shaped, the ones causing an issue were more "U" shaped. Gene
