i also am wondering if the plate you welded to the back side of the impeller blades on the pump threw it out of balance and caused a vibration. or perhaps the adapter you made had the fan out of balance or had it off center slightly?
The water pump that had the plate welded to it was not the one that failed. After welding on it the seals leaked even though I had cooled it between welds, so I had replaced it with another 8 bladed impeller pump.
The plate does not or has very little effect on the balance. I had a six impeller pump on the car for years. As a matter of fact it will probably be the pump I put back on.
The pump shaft shows signs that the hub had been spinning on it for quite some time.
That may very well be why the temps were erratic at highway speed.
This is just more of my "if it weren't for bad luck, I would have no luck".