cabinet

I just purchased a 2000 Chevy Express 3500, 5.7L. The instrument cluster was not working properly so I replaced it with one from my old 1999 GMG Savana 2500, 4.3L. The temperature gauge was not working so I replaced both engine temperature sending units and the thermostat in the Chevy. The temperature gauge works now, but is reading high, and higher than it did when it was in the old GMC. The gauge now goes up to almost red, then returns to about 5/8 or about 220 degrees and stays in that range. I had Pep Boys do a computer check on my engine and they showed me it was running at 195 degrees, while my gauge reads 220 degrees. The Chevy dealer assures me that both temperature sending sensors that control the gauge in a GMC and Chevy are the same. No one seems to know if the cluster gauge might be calibrated differently for different vehicles, or my gauge is just faulty, and reading incorectly. Or perhaps the new temperature sending sensor might be faulty. I might just have to hook up an aftermarket gauge or live with the difference in the reading. Any thoughts ?? Thanks.