Is the drop at the comb and heel the same as your pattern stock?
This will significantly alter your POI. You need to raise the comb so you see more rib. If your shot pattern is on left to right, it is either your pitch or drop that is off.
Another thing to remember is O/U tend to shoot high so they are set up with more down pitch than a side by side is so I would not compare the two for stock dimensions. Also pistol grip verse simi pistol grip will also alter length of pull slightly.
I would try adjusting the pitch first by adding some wedges made of cardboard or washers under the bottom half of the recoil pad first before I would start cutting. By changing the pitch of the pad you should be able to raise the pattern. Once you get the pattern on, you can shape a pad to the angled dimenssions.
