Vewry tall shooters face a number of problems. Ignoring your present vertical eye alignment with the rib, lowering the recoil pad far enough would allow shooting with a naturally upright head and neck posture.
This, as suspected, would probably cause cheek slap because as was noted, the pivot point on the recoil would be so far below the axis of the bore.
A custom stock would be the best bet but also the most expensive. Another solution as you mentioned, would be to install an add-on rib and raise the comb enough to compensate for the taller rib. (The adjustable comb would need posts that allow raising it the necessary distance.)
Not only would that allow you to shoot with an erect posture, it would not increase barrel rise (and cheek slap) during recoil. The challenge will be to determine how tall an add-on rib would need to be. Visit http://www.addarib.com for an example of add-on ribs.
It is possible that add-on ribs are not tall enough to allow you to shoot with a naturally erect posture. If they are not, you could use a pad adjuster to lowr the recoil just a little to allow an erect posture.
You could experiment with the necessary rib height by taping a strip or strips of wood to your existing rib to raise it. Balsa from a hobby shop is the easiest wood to work with but any wood would do. When you find the correct height, an add-on rib of that height may be able to be ordered.
The goal as you ralize, is to be able to shoot with your head and neck naturally erect and with your eyes in the vertical centers of their sockets.
And while you're at it, check the pitch on your stock. The wrong pitch is a common cause of cheek slap. The pitch is correct FOR YOU when the "whole" pad, "top" to bottom, makes simultaneous contact with your shoulder as you are mounting the gun.
Also, it would be best if you mounted the gun with the top of the recoil pad extending above your collarbone far enough too prevent its slippind down during recoil. That would be close to an inch above your collarbone. This aids in shooting with an erect posture.
Rollin