Rifle sights are not a handicap. As to which slug, there's no way to tell w/o actually testing them in YOUR shotgun. Since it's a rifled bbl you're talking about,probably one of the sabot loads with give the best accuracy. All 12 ga slugs are effective when properly inserted.
Something I've long wondered about is why folks want to push the envelop on slug hunting. The places I've hunted here in Md are probably representative of most Eastern Whitetail hunting. A long shot is 50 yards, I think my longest shot this last decade or two was about 65. IIRC, the last 10 deer or so got hit at less than 50, average shot last season was about 35. 4 deer, two M/L tags, one Bow tag, and one Shotgun.Still eating the freezer...
I admit the occasional long shot happens, but lots of those fall into the Hail Mary class. Bet the average killing shot in the East runs less than 50 yards. Heck, maybe 25% of my checkins are taken within 50 feet!
After bench testing, pick a slug and practice until you can put all your shots into a 6" group under field conditions. That's your max distance for clean and humane kills, with short bloodtrails....