Using a sawsall will be a certain mess on your head. All that
vibration will certainly bring down an unwanted tidalwave of chimney
buildup and metal chips. The plasma cutter will be a better option for
tight quarters like a chimney. You make this liner insert sound as
though you are planning on doing it yourself? It is money well spent
to have something like this done by a service provider that is
experienced, especially if you dont own your own plasma cutter..
After your damper is removed you may want to consider a Chimney
Balloon or some type of chimney plug to serve as your damper. You dont
want that open hole to be left unclosed to release your AC in the
summer and heat in the winter.