As Patrick said, the overheating is probably due to another reason. Unless you are running your timing very far advanced or very far retarded, that is unlikely the cause of your overheating.
Regarding timing: Find the knee. With the car in neutral, run the engine about 1000 rpm (a fast idle) and slowly advance the timing one notch at a time. The engine speed will increase until you reach a point where the speed will not increase with an additional notch. Move the lever back one notch and that is where you should be running the ignition for most of your driving. Make note of where the lever is. If cruising on a level road at a constant speed, you can advance the timing one notch. If going up a grade with the pedal to the metal, you can retard the timing one notch.
The reason this works is that the air/fuel burns quicker when the engine is running at a higher speed so advancing the ignition is not necessary. This is especially true with higher compression heads and not true with overhead conversions without a squish area.