Have a 1966 GTO 389 borde .030 mild cam, new flowkooler water pump, fresh stock radiator, new fan clutch even tried constant on elec fan. Temp goes to 245-260 in 4 to 5 miles. Has Edelbrock Performer RPM and Carb. Any ideas    

What is the timing set at and what thermostat are you using.  

Have same problem and can't figure it out. I'm starting to think that my block is clogged up somehow.  

Hi
Did you put the plates and tubes back in behind the water pump how are rubber seals on the ends of the tubes?? Is the Radiator a 4 core unit? Spring in the lower hose missing? causing it to crush shut from the suction? Is the Timing set correctly? they like 10 to 12 degrees lead with good gas. is it running lean?.. see if the plugs are white. was wondering if you can put a shroud off a 67 gto on it. might help if it aint got one. fan should be half in half out of it. tried replacing the thermistat? l have gotten bad new ones.. boil a pot of water on the stove and see if it opens .. maybe try a flex fan.. but if the clutch is working on the one ya got might not make much of a diffrence. my friend put a Aluminum Rad in his 67 and it cured it totaly.. expensive though.

                               Good Luck

                                  Tom  

Have defuser plates in  tried 2 stat's, tubes?  

I run a Flowkooler. The first one I had, had the wrong impeller. It only worked if I ran it backwords. I found out on the dyno. Sent it back and they were amazed it was like that. They sent me a new one and have had no problems. Make sure you have flow through the radiator.  

Have Plates in and have tried 170 and 180 high flow stat's  

What is the timing set at and was the block flush out when it was rebuilt if not unsrew the 2 drain plugs 1 on each side and drain it if nothing comes out then it is plugged up and you will have to stick a screw driver in the hole to help unplug it.Also what kind of carb is on the engine.  

Does the carb. matter? Why?  

If it has a factory carb. and factory thermostatic air cleaner  the (thermac) system maybe malfunctioning witch draws hot air off the head by the exhuast manifold..