Well practiced Karate chopping!
Sorry couldn't resist.
What is the pad made of? Does the pad hold the curve once removed from the stock? If it does hold the curve, you've got a template you can use to match the newly cut area to.
If you've got a power miter saw, you can take the existing stock (removed from the gun) place the comb against the fence then free wheel the miter angle until it's touching the toe and heel of the stock. Note the angle on the gauge, better yet - lock it down. At this point you can slide the buttstock along the fence till you've reached the desired amount of length reduction. Now cut the stock, and drill it to accept the recoil pad screws. This will take a bit of careful layout and measurement. Now attach the pad to the stock. As you predicted, there will doubtlessly be a bit of extra material hanging out around the perimeter. Wrap the stock with copious amounts of masking tape near where the pad is attached. A stationary belt sander will allow easy and square cutting along the perimeter. The aforementioned tape will ensure that you don't maul the wood whilst grinding. Of course if the pad is leather, you might be able to just cut it with a sharp knife. Some folks contend that the best method of pad fitting is to glue a sheet of paper to the wood facing side of the pad, attach the pad to the stock and trace the perimeter onto the paper. They then remove said pad, attach it to a jig or fixture and use whatever grinding device suits their fancy to reduce the perimeter to the traced profile. Personally I find both procedures to be irritating and painstaking. Rubber is a surprisingly difficult material to machine. Some types require that you oil your abrasives to reduce loading. Some types require that you freeze the rubber to get it to cut properly. All types will test your patience and desire to leave professional results. Good luck.
