runs hot at highway speeds

[PEGLEG_1]

I have or rather my son has a 78 camaro with a stock crate 350 in it with 350 turbo trans. He was headed up 75 and the temp gauge climbed to about 240 to 250 degrees. He got off at the nearest exit and checked the water level and had to add some. When he took off again the temp dropped down but climbed up again when getting up to speed on the highway. He say the temp is ok driving around town but runs hot out on the highway.

I was looking for some tips on troubleshooting this.

first could it be a bad water pump and or the clutch fan is weak?