I had that exact problem with my 67 Cuda. Previous owner had installed
all new front suspension. I too thought it was in the Pitman arm, but it is keyed in 4 locations, no other ones were close.
I removed the steering box to install a Firm Feel box, and installed a new Pitman arm, counted the turns lock to lock and still had the same problem.
The tie rod ends were turned in too far on one side, and out too far on the other. Car always went down the road straight, but I had to be careful when trying to turn sharp to the right.
Problem solved. Hope this helps...Larry