Caster is challenging to understand because it's set up based on how the car will be moving at speed. If you could closely study the front tires while they are in motion, you'd notice the tire contact patch is different than when the vehicle is stopped. In motion, the tire contact patch moves rearward, which changes caster. Caster becomes more negative in degrees once we get going. This becomes even more apparent with radial tires, which make the car wander more from side to side. The solution is caster adjustment whenever tire type is changed from bias-ply to radial or vice versa. In fact, it's a good idea to have a front-end alignment any time you rotate or replace tires.
Mustangs like 0 degrees of caster, meaning both ball joints are 12 and 6 o'clock from one another. Ford calls for a minimum of 1 degree of negative caster and a maximum of 1-degree positive with no more than 1/2 degree of variation between the two on V-8 models. In short, caster must be within 1/2 degree from side to side for best results. Six-cylinder models need 0-degrees minimum caster and 2-degrees positive maximum due to the front-end weight difference.
First-generation '65-'66 Mustangs caster adjustment takes place via shims at the upper control arms. Again, shims are added or subtracted to control caster. From '67-'73, the strut rods were adjusted to change caster. The same can be said for '74-'78 Mustang IIs, which also use a strut adjustment to change caster. The Mustang II's strut rods are attached to lower control arms from behind.