fireplace insert install question

I will be installing wood burning fireplace insert soon. Have never done this before, and need some advise on couple of things.. Have been looking for 20' X 6" stainless flex liner kit. So far Chimney liner depot is best price I have found....$370.00 for basic liner + terra cotta top plate + appliance connector + rain cap - all stainless steel - price includes shipping. are there any better options?
The liner will have to pass through the flu damper opening that is 4.5" X 20" so they are saying to ovalize 18" (18" is distance from damper to top of insert) so it will fit throught opening, then reshape to round then connect to appliance connector and to top of insert. The sales person I am talking to also advises me to purchase this material to plug hole where damper plate has been removed - http://www.chimneylinerdepot.com/store/903/product/No-Foil-High-Temp-Insulation-Section-24x48.html - also to wrap arround top of liner then put on terra cotta top plate. The existiong Terra cotta liner is 7 5/8" X 11 5/8" inside, so not possible to insulate whole liner. Sales person tells me it is most important to plug hole arround liner at damper opening. There is a large area between damper and the beginning of existing terra cotta liner. It is about 30" tall by 20" wide at bottom (narrows as it goes up) Menards has large bale/roll of non combustable insulation ( this insulation says non combustable per ASTM E136 on it ) I was thinking this might work better to shove up and fill the whole area. Does this sound like good idea? Menards clerk says this insulation is rated for 1200 degrees......but it does not say that on packaging. I have not been able to find out what means - non combustable per ASTM E136. From what I have read, the hot gasses from insert can reach temps of 600 to 800 degrees. Any advise appreciated.
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On 11/21/2011 8:12 PM, snipped-for-privacy@theShire.org wrote:

http://www.chimneylinerdepot.com/store/903/product/No-Foil-High-Temp-Insulation-Section-24x48.html
main question i would have is why are you using a liner?
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Steve Barker
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