Overheating

I have an 89 IROC Z in cherry condition, when I bought the vehical in 91 with 31,000 miles it ran fairly cold (just off the 100 degree mark).I only put 30,000 miles on it up til 1998 when I went overseas for work. The car sat for 7 years when I finally got it out the engine ran horribly (rings were toast) and needed to be rebuilt. Since the rebuild the car runs very hot almost always reaching the obverheat mark of 260 degrees. Ive taken the car back to the dealer 3 times and replaced the thermostat with a 160 degree, the radiator, the head gasket the coolant and it is STILL running much hotter than it should and ever did befor ethe rebuild. Im sick of throwing the dealer money and dont know what to replace next but want it to run cooler.