Hi, I have an oil fired trailer furnace in my workshop. It goes into a block chimney through a 6" thimble through the wall. Below this is an 8" thimble that runs through the wall. Both run through a brick section of wall. I am going to remove the oil furnace, and put in a wood stove. I would prefer to run the wood stove through the 8" opening, how should I plug the 6" opening? It worries me a bit since the 6" one is the upper one.