has it always done this or did this just happen? thats the first question........if its done it from day one then I would say the head gasket/gaskets were istalled wrong, what year is block and heads (351w heads,late model heads?) if not explain in detail when and how it overheats and we can figure it out........sometimes using late model and early parts or 351w heads on early blocks require water passage mods on heads.

 

My 289 did the same thing. Put the car on on an incline, nose up. Srart , let warm up, stop engine. Open the rad cap [ use gloves etc] this will burp the system. Do this a couple of times. Cool down and refill with antifreeze between 'burps'