Aftermarket temp sending units do not have the correct resistance curve for our gauges. The GM #513321 sending unit hasn't been made in 20 years, and the cross-reference replacement doesn't have the right resistance curve either.
The #513321 sending unit is the correct one for our trucks for all engines. The sending unit has to be matched to the gauge to get the right reading; what engine it is in doesn't matter. It is the resistance curve of the sending unit together with the gauge that have to be matched.
When you previously saw the needle at 210, the temp was actually lower, probably because you were moving and the radiator had a lot of ram air through it. When you saw it much higher, it was probably running at full operating temp. With the wrong sender resistance curve, though, it will read incorrectly all the time.
So, what do you do? If you got a couple hundred bucks laying around you don't need, you can get a NOS #513321 on ebay. Or you can order a reproduction unit with the correct resistance curve from these guys. They even have the right connector. I put them in my truck and now my gauge reads dead on with my laser style temp gun.
http://www.americanautowire.com/shop...ding-unit-5028
http://www.americanautowire.com/shop...ure-sender-con