Palmer
Find an eye doctor that knows something about shooting glasses, not all of them do. My eye doctor is a competitive shooter. Take your double rifle to the eyedoctor, he should be able to make you a pair of shooting glases that will work. Shooting glasses are set up different than regular glases, they should set higher on your face and the optical center is adjusted for your shooting stance and head position.

I recently handled a Manton 470 double that was fited with a peep sight and it looked like it would work great. See it at www.champlinarms.com

When/if I can no longer shoot my bigbore doubles with iron sights I will either scope them

[I would probably never scope the 450 No2, but I wish my 450/400 31/4" double had a scope on it.] or just get a Chapuis 470 and scope it.
I have a scope on my Chapuis 9,3x74R in QD mounts and really like it. I would have to do the math but I have shot about the same amount of game with iron sights as I have with the scope. I have made several shots with the scope that I could not have made with the iron sights.
I have killed several animals at 100 to 200 yards, a coyote at 271 and a Kudu at a little over 300 yards.
I have also made some shots "in the dark" ie pigs, a side striped jackel and a civet cat. Also I consider the scoped double the best choice for bear, leopard, and lion over bait. Well actually I consider the double rifle the best choice for all game under 300 yards.