[68gmsee]

I have a 250 in mine and I also used to have a heating problem after I rebuilt it.
There were no head gasket leaks or other problems with the engine that would cause it to run hot.

Things I did on mine:

I bought a temp indicator cap at Auto Zone to make sure it was running hot and not the gauge being wrong.

Then I checked all of the hoses and they were all good.

Made sure the t-stat was working properly. Tested it in a pot of water with a cooking thermometer and checked that it opened up at the stated temp. It did.

Experimented with percentage of anti freeze to water. No help.
Replaced the radiator cap with the recommended one. No help.
Replaced the water pump. No help.
Had the radiator rodded. No help.
Installed a double core radiator. No help.

As a last resort, I installed an overflow container to prevent fluid loss and that finally fixed it. Temp stays at the normal level even idling in the hot days.

Not saying it will fix yours, but it's a cheap way to find out and/or prevent fluid loss.