I had a similar problem though, my gauge didn't read quite as high. I had a 350 Olds in a 63 C20 and the gauge would read about right most of the time. When I put it under load on a long hill or the weather was hot, hot the gauge would start to climb. I flushed the radiator ran coolant, ya da ya da. Finally put in a aftermarket mechanical gauge. The added gauge would read 20-30 degrees less than the stock gauge after about 160 degrees. I never did replace the sending unit, I may have replaced the temp guage trying to find the problem.
Larry
63 C20 292 daily driver
65 C10 Custom in process
