Sometimes you may never find the problem even after replacing "everything." In my GTX I rebuilt the carb and all jetting, air intake, water pump, recored 3 row 26" factory stock radiator, fan/clutch, TStat, shroud, hoses, belts, pulley, fuel pump, all high voltage delivery items, using coolant of 90% demin water with Redline water wetter, optimized timing adjustments, and even putting the cam shaft back to 100% factory stock to reduce engine power. Never solved the problem of overheating at idle on hot summer days. Lived with it for 3 yrs before selling the car. Used an overflow container to prevent leaving puddles at shows. The only step I didn't take was an oversized aluminum radiator with a much higher heat dissipation.

If you run with the heater core at full bore and it keeps your operating temp a bit lower than when it is off, you probably have decent flow through it. If your radiator is gunked up you should be able to see signs of blocked channels at the top of the core.