I had a similar problem but different aspects. I had a pull to the right and took my street rod to a shop for alignment - They charged me EXTRA because the had to be so CAREFUL with my car. It worked great but a little time later, it started pulling again. I ended getting a Caster/camber guage setup for home so that I could check it and it was off. It turned out that my A-arms were heavily painted/finished and the teeth had no bite. I used an air cutoff wheel and recut the teeth in the A-arm. Reset the caster/camber and it's been driving like a dream ever since (2 years +). It was worth the guages that I bought at Summit.
Jerome