These were common problems with the 534's at the end of their manufacturing run. It is good advice to check for restricted exhaust, tune up, ect., but you also should look at the air pump system if it has one. Too much air into the exhaust will cause the manifolds to glow. Another thing to look at would be the carburetor. I doubt that it's an 1150, it sounds like you have the stock one on there, which I think is around 600 cfm. If the power valve is blown, it could cause lack of power, and over fueling (which also could cause the exhaust to be too hot if the truck has an air pump). Most C series trucks had an intake fresh air/heated air system, check and make sure the engine isn't getting hot air all the time.