Most times I find You can only get about 2.5 degrees with the bushings. You can shorten the strut rods 3/8" which will help. Or what I've seen on some S/S cars is moving the k-frame attaching nuts forward. I've seen them with 7 degrees. Remember chassis rake will effect your caster number. The higher the rear of the car , the less positive caster you can acheive. My 64 had only 2.5 degrees and tracks fine at speed. It also has nearly 3 degrees of chassis rake.
Doug
Doug