This is my second time typing this because I was going to suggest the scope mount first. Then I decided that was just because I like scoped slug guns. For shots under 75 yards I think you should just look for a used smoothbore slug barrel with rifle sights. With the foster type slugs, this combo will do everything you need it to do. You should be able to find a used smoothbore barrel fairly inexpensive because the rifled barrels are so popular for guys trying to extend their range with a slug gun.
If you can find one, the rifled sights/choke tube Remington barrels make good combo turkey/deer guns,but they do get pricey.