I scrolled thru the thread and saw a lot of good suggestions. However, I saw no mention of how many miles the engine has, have you ever checked to see if the timing chain is loose? I've had slant sixes, small blocks and big blocks where I've chased everything and nothing cooled the car down on long cruise until I changed the timing chain and gears. Putt around town all day and be fine, try to get up to speed on the interstate then the temperature gauge rises. Pop your distributor cap, then rotate the crank slightly by hand, then rotate it the other direction. If there's much lag at the distributor rotor at all when you reverse direction, the timing chain has too much slack and needs replaced.