I would take a look at the steering box. The 75 Duster I used to have did the same thing, wander all over the road, and the guy that did the alignment told me that the box needed rebuilt but he wouldn't guarantee it to last more than a couple of years and he'd been in the business for some 40 years. I looked in a few junkyards for one and couldn't find one and I don't think it was because they were so popular. I had intentions of changing it to r&p, but sold the car before hand.