You need some metal shim stock or heavy protective tape to protect the stock finish when grinding the pad flush with the existing stock. A cutup aluminum can works for this, or you can use double layers of duct tape.
A bench-mounted power disk sander or a belt sander works best for this. You hold the stock firmly, and angle the stock w/pad to cut only on the pad portion. Make the pad look like an extension of the stock wood. Avoid marring the stock, of course.
Actually, you don't need it to be perfectly flush to look good. If you look carefully at factory stocks, you will see they got that way because the pad fitting was done before applying the stock finish.
HTH
John