The exhaust gas test would be a good next step, my brother found a leaking head gasket on his Jeep with one of these. It looks like the engine is running around 210*, your tests confirm this, radiator is doing what it can, but only removing about 20* of heat. I don't know if thats good or not, never really looked into how much they should lower the temperature. Whats the tubes look like in the core? Your mild acid system cleaner might help if they have scale build up.
It also sounds like the engine is pretty clean inside since it flows out the drain. It seems that area of the blocks is where everything settles. Back flushing won't hurt. Maybe come up with a water and air pressure nozzle of some kind if you don't have access to a power washer, a piece of 1/4" brake line on a air nozzle that you stick in the block each direction with water coming in from the front of the block, at a minimum a coat hanger to pock around in the block.
It also sounds like the fan and pulley are right, a shroud would probably help some, but I doubt it would cure it. It worked as is for decades, so something is not right.
Try the test for exhaust gas.
