I had a 327 with the same thing timing may help a little if is set wrong but seriously if you are cruisin for half an hour and your temp is ok but when you stop in idle or in traffic and you temp rises you my need to fit a well fitted shroud to radiator. After you have a shroud fitted and it only slightly helps you may try different water pump pulleys to change water flow pressure.

I tried running twin thermos on many different combo's on a 5 core radiator and it wasn't until i threw them in the bin and built a shroud thad i had success.Also another combo is to fit a twin themo's shroud unit out of late model fairlane,if you dont want to run a fan at all but an incorrect fit will result in loss of fan suction and result in OVERHEATING WITH NO VEHICLE MOTION