Barrels with ribs make for easy placement of rear sights.
: I simply dovetailed mine in. I've cut a lot of sight dovetails in muzzleloading barrels so it's no trick for me, but for someone who has never done one, I'd get a 'smith' to do it for you. You don't want to 'scratch' those tubes with the hacksaw or files. The front shotgun 'bead' may or may not be too low, depending on where the gun shoots. It can be filed on the sides to narrow it a bit, not much is needed, if at all.
: I picked up a couple boxes of WW slugs a couple years ago and only tried 2 from the pump gun. I didn't have any balls loaded up and was going to camp. They were the higher velocity ones, 1,700fps, but only 1 ounce. The 2 shots I tried, made a single hole at 25 yards, some 4" over my bead, so I left the shotgun at home. 4" over is too high and there was no time to 'adjust' things(higher front sight). In the dark on a facing bear it's difficult to judge holding 4" lower than where you want the ball to hit. Time is a factor. When there isn't any for shot placement adjustment, bad things can happen.
; Naki - a bit of windage adjustment and bingo on the Brenn's. They're just a bit wider than distance between the muzzles. That could show minor spreading or normal group size - Cool! More shooting will show what's going on. I find it interesting the WW made larger holes than the Brennekes. I'd have thought the Brenn's would have made larger holes with their wad-cutter-type ogives.