'75 Duster steering issue

+2° caster isn't very much, especially for power steering.

The length of the pitman arm is the same for both small and large sector boxes. The primary difference other than the diameter of the sector was the direction the ball joint on the pitman points. If you only changed the pitman, you must have had a 73+ manual steering box pitman on that small sector power box. Otherwise you would have had to change the center link as well, because you can't use a 73+ center link with a 67-72 pitman arm, the taper on the pitman arm mounting hole runs a different direction.

Pictures would help. Otherwise, I would check to make sure that the steering moves back and forth easily by itself. Jack up the front end and turn the wheels manually and see if you feel any significant resistance. If you do, you'll need to look for bad components (idler, tie rod ends, ball joints, etc). If you don't, it's probably just the low caster number.