on the early cars ( and im realy talking mk1 cavs / mantas ) etc the bimetal dashboard voltage regulator can fail and this makes the guages read high . nof experienced it on later stuff as they use an electronic version .
so at least then you can confirm that the guage is reading correct and go from there .
usual suspects ( as im sure you know ) for overheating are thermostat(eliminated),water pump(eliminated),head gasket(eliminated)
electrical/sender/voltage reg/guage(soon to be eliminated/checked)
after that you are into
cooling fan
head or block damage
head or block blockage
hopefully not to depress you but we had a similar issue once with a 83 carlton 1800. head gasket was done , thermostat, pump etc and we never got to the bottom of it . akways went up into the start of the red but never further and never boiled but nothing we changed brought it down !
valve timing
ignition timing
water in fuel combustion issues !