rear gear change advice

[PxTx]

I would have expected more than 15 degrees difference between the upper hose and lower hose. I would focus on why that is and hold off on looking at the block. I think you are on to it with the clutch fan being loose after a shut down heat soak.

Friends in California run straight water for better heat transfer. You should run something like the water wetter or other conditioner to help the bearings in the water pump and keep the rust down. Antifreeze is just for that. You could run water in the summer and drain half of it and add Antifreeze in the fall.

There is also a great debate on the benefits of Evans Coolant, but I don't want to muddy the waters in this thread.