caster question

My '65 Barracuda started wandering all over the place at high speeds on the highway and the strip. after I got an alignment. It has manual steering and a 360 engine. I took it back to the tech and he said he couldn'tget the caster to specs. It's sitting at about 4 degrees negative right now. So I have installed adjustable strut rods so that I can get more positive caster. How far positive do you all think I should set the caster to get this car going straight again?