My car was doing the same thing.I , like you checked everything. Except one day i was looking at the throttle cable, i noticed my carb to intake manifold bolts were loose. tightened them up bam temp dropped 20 degrees!make sure you check the simple stuff.

The only thing i'd add is if you have a 180 degree thermostat high flow. I dont think you can swap the hot water in the block with the cooler water in the rad.too fast the hot water will exit fast giving it more time to cool in the rad. the cold (relative) water coming from the rad will hit the thermostat and close it.

But being in a hot environment running a 160 thermostat and your car runs @ 200 + at the rad and block it will never close. it will remain open for the most part and just act like restrictor in the flow.