Just Started up my rebuilt 440, everything is looking good except I had to shut it down after 8 minutes due to overheating. The temp got up to 200 before I shut it off, the upper hose and very top of the radiator was hot and full of pressure the rest of the radiator and the lower hose were cold. My first thought of course was a bad thermostat but I was wondering if it could be air bound? Thanks for any help!

Could have very easily been air locked. I always(when it is do-able)fill engine through thermo opening till it's about to spill over, then install the thermo and cover, top off rad a little , then start it.

I also leave the cap off so it can burp when the stat opens and leave some room on the top for expansion and save a little on mess.

I'd prefer to keep the stat out for the breakin. Your timing is in the ballpark? A shop fan in front of the rad?

Timing is close but not yet checked with a time light. I have the stock fan and shroud on

Radiator could have been partially blocked and now you are building heat with a fresh engine. If you can run your hand across the front of the radiator safely while the engine is heating up and find a spot where the radiator gets hotter then cold you have a blockage. You have to do this before the whole thing heat soaks and is under 120 degrees to use your hand. Works on trucks every time. Also with the cap on if the lower hose seems collapsed and top has pressure at a higher idle same diagnoses. Water pump draws from the bottom and a blocked radiator will cause a restiction vacume.