i think it could potentially be in the tune. i went from 1/2 - 3/4 gauge reading down to 1/4-half gauge reading with a recurve dizzy new carbs and a better exhaust tuned up right.
Or a problem with the stat or cap
Or not necessarily the cap, the lugs on the cap and the filler spout profile have to hold the cap down tight. bend the tangs if need be.
or the wrong cap. there are two common spring lengths on caps for mopar radiators
and the short spring cap on a long filler spout causes overheat Cap may say 7lb or 16 lb. that means nothing if the spring is too short, it won't seal the system. its like running with no cap on, no pressure in the system causes boiling in the head and you will run hot and the hood will be damp above the cap.
Dave