Highway heat

i had a 68 with a 440 ,did the same as your car, ended up switching to a 195 thermostat and driveing down highway it would stay 195
so for a quick switch of a thermostat i would try that first, it fixed my problems,
if you think about it the thermostat stays closed til temp gets 195 then opens so coolant stays in radiator longer to cool , 180 opens 15 degrees sooner,and cant cool radiator down.
atleast thats how i look at it..