Generally there is not much wood under the RR hole when the gun is finished. It looks like you have somewhere in the neighborhood of an eighth of an inch, perhaps a bit less. That will work, it is generous even. You will want to remove that "step," and reestablish your centerline, before you get to shaping the forearm. A #4 Stanley hand plane would be ideal for shaving off the excess wood, a coarse rasp, or a band saw would also work but be extra careful with a band saw.
For those asking; the stock is ash.
dp