Re: 8 inch to 6 inch stovepipe
I would choke down your wood stove at the stove, just like any other adapter you might install on the exhaust.
I've done much the same with a 12 cubic foot stove of mine. I narrowed the exhaust from 8 inches to 6 inches, and I have had no problems. The stovepipe rises about 12 feet, so drawing out the stove isn't a problem at all.
My 8 to 6 inch modification allows plenty of air to enter the stove. I can still get it orange hot if I open up all the vents and let it rip.
Nevertheless, keep a close eye on it for the first few fires since you never know where Murphy might be lurking. It is always better to be safe rather than sorry.
I test all my stove modifications outdoors so I know they work well before I reinstall. Since your modification is just necking it down, not installing modified air intake, you'll probably not need to run a full scale test outside.