The back of the head sensor port is the hottest spot in the cooling system and 200° isn't that uncommon. If you have 50/50 coolant it will raise the boiling point by at least 10°. 200° is till quite a way from the boiling point. And the gauges aren't that accurate. The big thing is to keep the thing from boiling because that is what causes damage. I have been operating over the road trucks for over 60 years. We do things a little differently now than we did back in the gas GMC days. We have master gauges to check critical things such as temp oil pressure. If the temp worries you, use a known correct gauge on it and see what you have. Back in the day if we cracked a head, we just went out and got a different one. GMC heads aren't that plentiful any more. What kind of radiator do you have? if you have the one that cooled the original 228, it will be borderline on a 270. Back in the day we always installed radiators out of trucks that had a 302 to keep engines cool in Texas. They bolt in, but have thicker cores. I like aluminum radiators. They transfer heat better than the old brass ones did. But if you buy one, make sure it is for a GMC. Lots of vendors list radiators to fit Chev or GMC. That is not the case. The mounting brackets on a Chev sit at a different angle and the filler is on top of the tank. You need one with the filler neck like you now have.
