i had this problem on my 68 charger. it would run hot on the highway but would be fine in town,
i went from a 160 thermostat to a 195 and problem was fixed..
if you think about it a 160 stat will stay open almost all the time and not let the coolant stay in the radiator long enough to cool down, so by putting a 195 in it will probally fix the problem..
its a quick fix i would try first
