Several questions about this and maybe we can help you more.
Are you scrapping out this stack, or are you going to be putting it back up?
Is cutting holes large enough to put a beam or large heavy schedule pipe that will carry a load of this size through the holes, and toggleing the stack, then welding lifting lugs to the pipe, beam, square tubing, etc. a possiblitiy?
I know you know as well as the rest of us, you have to know what the material is if you are going to weld on the material, and even more so if you are going to be preforming a critical lift such as this. Someone getting hurt or worse, is not worth the guessing game. If the PMI readings are varying, I guess I would go to what is listed on the prints.
7018 might hold it, but the key word is might. Having both performed and supervised lifts such as this, I guess I would not chance it. There are too many things that could go wrong and once you have made the final cut, there is no turning back.
Have a nice day
Mike