Getting the pittman arm off the rocker shaft can be a chore. Last time I took one off was on a 100, but this might work on a 6-cyl car: Loosen the shaft at the collar under the fascia, unbolt the steering box and the tie rods from the pittman arm, and turn the shaft/box 90deg anticlockwise. Then, you should be able to get a puller on the pittman arm.
As I mentioned, I haven't tried it on a 6-cyl car, but it might be worth a try. On my 100, I was able to unbolt the radiator and push it aside to make working room (IIRC, the shop manual says to do this). You may need to loosen or even remove the damper as well. Now that I think about it, you may not have to remove the pittman arm before pulling the box, but you'd still probably need to rotate it.