I believe the 10 ga and the 12 ga 3.5" models used the same receiver in the BPS. Aimtech or some such are makes a saddle mount for the 3.5" BPS which might work with no drilling.
Otherwise, drilling and tapping holes on the top of the receiver is not difficult to do, the hard part is keeping them in line with the barrel. I've owned many rifles that were drilled and tapped by a "gunsmith" later on - the Millet adjustable bases became my friend if the gun did not recoil greatly.
I would much rather go that route, but haven't found any information saying they are the same
