I don't know where you got your pulleys but CVF Racing offers a high flow pulley. I believe it's 6.4" diameter.(if you want to go back to an aluminum pulley) Not as large as the one you're trying now though.

https://www.cvfracing.com/chrysler-small-block-pulleys-s/44.htm

I've read that some electric fans can be a restriction to air flow at cruising speeds.(while they are running) I wonder if hooking the fan up to a toggle switch and shutting it off while on the highway would be a good test to see if the temps stay lower.