Bypass the radiator.
A rebuild is not going to fix it.
3 ways to remove heat from a transmission
Larger cooler/cooling fans
More fluid capacity (larger pan)
Reduce slippage. So better converter that slips less and locks up more aggressively.
Valve body that reduces slippage between...
The issue is in high gear you're not fast enough to keep the converter locked up and this creates heat. I would talk to wholesale about a valve body. This will allow lock up in third gear.
8000lbs is a lot too.
I had issues with trans temps in the past. I'm curious to see how my monster...