What's wrong with keeping your 870 as a dedicated deer rifle, since it already has a rifled barrel. I have an 870 with a rifled barrel and cantilever scope mount. It wears a Nikon Pro-staff 2-7x 32mm scope, and shoots Hornady SST sabots really well. I shot this 200 pound doe right through the heart at 110 yards.
http://i243.photobucket.com/albums/ff75/huntingthenorth/P1010002.jpg