I have a woodstove with 6 inch pipe collar. I bought new double-wall 6 inch stovepipe from DuraVent, including an adapter between the stove collar and new pipe. There is a gap. The new adapter is smaller diameter than the 6 inch collar inside diameter. The old single-wall pipe connector was a tight fit. The stove is a handmade Sundance stove from early 1970's. What can i do to properly adapt the new pipe to the old stove collar? Is there an adapter available? Do I need a custom fabrication? thanks for your help.