These engines have a few issues with overheating.
1. bleed procedure.
when you replace the t-stat, bleeding air out of the system is very time consuming and frustrating.
you will see there should be 2 bleed valves. 1 near the waterpump tube and the other at the thermostat. there should be a steady stream of coolant coming out of these when running to indicate a proper air bled system. even 1 air pocket trapped in the engine will cause a overheat.
2. head gasket
these engines are notorious for the head gaskets leaking and causing pressure in the system so it overheats. generally, when the system tstat is ok, the cooling fan is operating properly and there are no leaks, it is usually the head gasket that is the culprit.
3. intake manifold leak.
the corners of the intake manifold leaks on these and then we get air pockets, which will lead to a overheat condition.
4. rad plugged.
this can usually be eliminated by feeling the rad when the sytem is hot. there will be cold parts on the rad if its plugged and you can usually see this through the rad cap opening excessive build up
so make sure the system is bled with no air, there is good flow through the rad. if it is still overheating after all of the main components are ok, than you are likely a head gasket issue
let me know what you find