I have a superior Heatform that i would like to cut through the "slopping" top plate to provide access to place a solid liner up an old tile flue. By doing this i should be able to use a new insert with a sloped 45* stove pipe connection, make a 45* bend and go straight up the flue!? Cutting out the heater pipes, damper, and excess material (as needed) is more complex and requires a very flex-able liner. After cutting the vertical access holes i plan on filling the Heatform inner body with Rockwool. Is this reasonable, safe and acceptable. Any additional input is appreciated.