You'll need to remove the little "ears" from the grip frame to mate up with the Montado. Other than that, it should be an easy swap (other than the lock mechanism aspects that is). I can't answer as to how the lock and main spring assembly will fit into the older grip frame.
If you put that birdshead on a Montado, you also need to order a mainspring strut, seat and spring at the same time. It will eliminate the lockign mechanism on the gun. This should not be considered detrimental.
The Power Custom version of the XR3 is specifically designed to be roundbuttted as well. http://powercustom.com/store/index.php? ... c7ff46aade
Some people might like the XR3 basis enough to spend the extra money.
You can also use one piece grips with it if you like.
(Still need new strut and spring for a Montado, but not seat)