I have a metal bestos chimney set up that I was using for my wood stove but am now converting over to use for my pellet stove...All the stores around are out of 6 inch to 3 inch reducers...They do however have 6 to 4's and 4 to 3's. Can I get away with doing the 6 to 4 then the 4 to 3? When the exhaust pipe comes out of the pellet stove it goes straight up through the chimney, there are no elbows...