If your read the Brenneke box it will say OK for full choked guns, although a more open choke is preferred. A foster slug can be shot from a full choke barrel, it's very soft lead, I know old timers that have done it for years. My FIL shot them from a Winchester 1400 for over 30 years, 28" barrel, no issue, after he passed my BIL added a saddle mount and scope, the gun cut a clover leaf a 50 yards with Remington's. Main thing is know your gun, a shotgun with foster slugs is like a .22, find what your gun likes...........Williams makes sights that will fit your rib.
The biggest issues you'll have with foster's is leading in the barrel, a deep clean will be needed, being a thimble shape they expand at the base for a snug gas seal. Brenneke slugs are made a bit under size for the bore, they have a wad system to act as a seal, as a result very little leading.........I hunt in counties that require shotgun for the past 58 years.
Your best choice is to go for a rifled tube so you can use Sabots and add sights to the rib.
