[Dan Lancaster]

I suggest that you recheck the timing. First check to make sure the mark on the dampner is pointed to the right location. To do this, take out number 1 spark plug and turn the engine to top dead center. I think I have seen a tool in Summit Racing or Jegs to help with this. Summit's number is 800-230-3030 and Jegs is 800-345-4545. Or build your own out of an old spark plug and a bolt. You have to break out the insulator, drill the metal hub and tap it for a bolt. The end of the bolt is screwed down into the engine so the piston strikes the bolt. Mark the dampner in this location and rotate the engine (by hand) in the opposite direction. Again when the bolt strikes the piston, mark the dampner. Half way between these to marks you made on the dampner is the actual top-dead center. Now retime the engine to spec.
In my earlier days, I had a Mustang which overheated when in the city. I would have to cool it down at 60 MPH. Turned out the timing was too far advanced. I found this out after several water pump replacements and many thermostats.
Hope this helps. Looks like you have tried everthing else. If you have any trouble with my explanation, let me know...