I saw that my chimney pipe was cracking, as I thought it might, so I've decided to replace it with Duratech double-wall stainless, insulated chimney, before I build the enclosure. The area on top of the vent isn't big enough for the anchor plate, so I'm going to cast a manifold to suit. I'm a little concerned about the "cold joint" between the arch and the new work, but I guess it'll be ok. I plan to drill some shallow holes and insert tapcon screws halfway in to help anchor the new casting. I'll also probably anchor the plate right in when I pour the cement.