I'm in need of some help. I want to change all this single wall out to double wall due to clearances. The flue pipe is Metabestos SS 11. I'm kind of confused because two stove pipe adaptors are listed. One is std. and one is flush mount. Also they show a telescopic and adjustable length. Another question- I already have the pipe adaptor so can I just use the finishing collar? This is alot of questions I know but I need some guidance. Thanks!