Hey SC. DO NOT retard your timing. Just the opposite. The more advanced you set it the cooler it will run. The flathead exhaust passages go between the cylinders and through the water jackets. Retarded timing means that superheated exhaust is turning your cooling system into a blast furnace. In most cases the cooling issues with a flathead are at higher RPMs for this reason. The best solutions for engine heating are:
1 Timing as high as practical (this is imperative on a flathead)
2 All air dams and shrouding installed on car
3 A full radiator shroud
4 good tall gear ratio in rear end. (keeps RPMs down).
There are other possible issues. Lean fuel, rust in cooling system, collapsing heater hoses, blown head gasket, Bad thermostats, etc but the above are the most common. Good luck.