1974 Duster Idler arm question,

Ron, Does the idler arm slide up and down on the bolt or wobble up and down?
The tabs on the k-frame do sit a ways off the idler arm, but the arm has a sleeve that the bolt goes through. Yes there are shims / washers that come with a new one to make sure it's level with the pitman arm.

There should not be an inch of travel. Are the tabs parallel to each other?

As for the k-frame being out of a different car, the Duster never came with a big block and the differences between /6 and v8 K frames are just for the motor mounts.
It could be from a different year. The 74 should have a center link with the holes for the idler and tie rod end in the same direction. Earlier center links had the idler hole and tie rod hole at 90° from each other.