Running Hot on the Highway

You didn't mention if you're running an auxillary tranny oil cooler and if so how is it plumbed, eg. oil goes to the aux. cooler first then the rad lower tank. With the slippage of the high stall converter (non lockup) you're putting a lot of thermo load into the rad. Go with the 180 deg. high flow thermostat and check it in a pot of boiling water with a thermometer first, 3 or 4 times open and close. I had one fail to open right out of the undamaged package. Sounds like you've done most everything right except pull one sparkplug and tell us the color of the electrode, white, tan, brown or black. You can also check to see if the lower hose is collapsing by simply have a friend bring the engine to the same highway RPM and see if it does, I'd also check it from underneath just don't go getting yourself run over by your friend.:D

Terry