I was going to wait until I finished rereading OT to make the upgrade thread for this and other stuff, but whatever. Since I have to wake up early tomorrow I'll give my arguments now and answer any questions/doubts tomorrow.
90% of the time, when Kamachi talks about nuclear shelters he's talking about Town to City level AP or Durability, it's a comparison he uses a lot in this series very consistently. It fits the Pope-class statement given in the same description, as
the literal Pope in the series absorbed most of the damage of an attack that would have destroyed the Vatican and part of Rome.
But there's more, Index's Walking Church was a super high-class magical item, a replica of the Shroud of Turin and one of the best defensive magical items in the planet. She is part of a faction that has flying magical fortresses with magical bombardments that are said to possibly have more raw power than a Saint like Kanzaki and could cause some minor damage to freaking Carissa with Curtana Original (so the Saint comparison is pretty believable) and with military forces that are said to be capable of wiping islands from the map.
Despite this Stiyl, a member of the same group and an expert magician who has worked with a Saint like Kanzaki in several occasions, considered Index Walking Church unpenetrable/unbreakable, "unless the dragon of St. George came back". Their plan to get Index back to get her memory erased in the first volume was even for Kanzaki to attack her and knock her out, since they didn't know the Walking Church was broken by Touma and thus Index would be in real danger
Said dragon is the basis of Index's offensive spell Dragon's Breath considered capable of breaking through the Walking Church. This is a spell Touma cannot fully negate all at once with IB and even managed to cause minor damage to his hand and eventually broke his pinky with the sustained beam. This is above what 99% of spells have managed in the series, including things like Saints or Curtana Original slashes. If this isn't a mark of high level of power for the series I don't know what it is.
Therefore, it's perfectly reasonable to have both Walking Church and Dragon's Breath at 7-B, because they should actually be 7-A or 6-C considering Dragon's Breath feats and the context of the series.
Unrelated, but I'll take the opportunity to add these ones too since they were also going to be a part of the same upgrade thread.
Leivinia Birdway scales to Saints since she can fight Kanzaki. She should be 7-A with magic, which would also upgrade Aleister and Samuel Mathers. We already know regular magicians can reach Saint levels of power with magic, we have
several cases of normal magicians on this level in the series already, so there's no reason those three, which are some of the strongest magicians in modern history, don't scale to that.
Base
Brunhild Eiktobel should be upgraded to Small Island level AP, as with her cutting spells her claymore could almost stand up to Kanzaki's Yuisen. It got chipped, but she still managed to block several slashes instead of simply being sliced into two like
Misha Kreutzev's wings. I initially put her as that when I first made the page but someone must have undone it.
