It's tough to recommend a slug. Different guns will like different slugs. I'd go buy 4 or 5 different slugs. Just the small box of 5. Shoot 3 of each slug, switching targets between brands. Whichever groups best, go buy another box and hunt.
I'd shoot at 25 or 30 yards with a good solid rest. This should give you a good idea of which slug works best for you. When you find the best group, move back to 50 yards just to see if you can shoot accurate from that distance.
I'd also go with 2 3/4" slugs. 3" or 3 1/2" slugs can be brutal to shoot. And you'll be doing alot of shooting trying to find the right slug.
Good luck.