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p.1 #9 · p.1 #9 · Panoramas: Dedicated Mounting Kits or a Pannig Base + Extendible center  
The first step in any panorama is getting the axis of rotation aligned in the vertical direction. That can happen by leveling the tripod, which takes longer, or by adding a leveling base to a ball head, or by putting the rotation above the ball of the ball head, and using the ball head like a leveling base. The Acratech GP is set up for inverted use, so the rotation is above the ball. Another way to add rotation above the ball is by adding a rotating clamp to any regular ball head.
If you put the rotation above the ball head, and you want to keep that level for a panorama, you now have no way to tilt the camera up or down for multi row panoramas. Using a L bracket on the camera is the perfect way to easily get horizontal or vertical orientation for panoramas, but it does nothing for tilting up or down. You need to keep the axis of rotation vertical while you tilt the camera up or down for multi row, and there just is no way to do that if the rotation device is above the ball of the ball head.
The only way to get near perfect correction for the nodal point and do that for multi row shots is to have a special rig that has both a vertical axis of rotation and a horizontal axis of rotation, and have the axis of both of those rotation devices point through the same point in space. That point in space is where the nodal point of the lens goes, and to get it there on most dual axis rigs, you still need the one nodal slide for a lens that does not have a collar. So perfect parallax correction requires the special dual axis rig, and there is no way to do it with a single axis of rotation set up, like with just one ball head that has a pan base.
If you put the leveled rotation above the ball with a ball head, you can only do a single level row of shots, and can not do any tilt up or down for other rows. If you leave the rotation device below the ball, like a regular ball head with pan base, and you level the ball head to get the rotation axis vertical, you then also still need to level the camera using the ball head in order to get the camera aligned with the rotation line. That is extra work that you either guess at using your eye, or requires another level mounted on the camera or in the camera. Adding a nodal plate will correct for parallax for the single row level panorama, but as mentioned before in an earlier post, if you want to tilt up or down to get multi rows, your rotation point is well below your lens, so multi row will not be exactly corrected. But depending on how far your lens is above the center of the ball, and how much you tilt is, your lens nodal point will not move too far away from the rotation axis.
Adding the nodal plate does increase the distance of the lens above the ball, and turning the camera to vertical orientation also increases the distance above the ball, so each of those make the multi row a little worse.
All of this discussion about maintaining a perfect nodal point is really not necessary at all for almost all single row and multi row panoramas. I still think you should just get an L plate and a camera mounted vial level and shoot things in both horizontal and vertical, and see how your panorama merges come out with the software of your choice. You will likely be completely happy with the results. If you have lots of close objects, and think you need to go to the next step, then add a nodal slide. If you decide you do not like to level the tripod all the time, get a leveling base and add it to your ball head, so you can still use the ball for your multi row tilting. Then, if you really want perfect parallax correction, and do not want to worry about the secondary leveling of your camera, and want to spend the money, add a two axis multi row panorama rig and level it to use it. You still will want a ball head for other uses, so that is not wasted. Most rigs would make use of a nodal slide if you had one by then. And if you had added a leveling base, that would be very useful for a full multi row rig.
Edit: Dave said it with a lot less words than I while I was writting, but the answer is still the same. I hope my long version is useful to others to help understand the details and the trade offs and the options.
