Forgot to mention the cap, that could be a problem too, and like the thermostat, is cheap to replace.

Another thing is the belt on the waterpump, I'm sure you've checked it, but if its loose that could contribue to overheating too.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi