six inch is good with the rubber and the bike clamps.
but 8 inch will just fit easy.
Since I had the bike clamps and the rubber I purchased the 6 inch mufflers.
the bitmaintech fans are about 6.75 inches on a diagonal so a 7 inch round hole works.
don't knock the importance of the rubber and the bike clamp as that decouples the miner from the muffler. My Hot air heat duct for my home has a rubber joint about 1 foot from the furnace it acts as a sound deadener. It is antoher reason why I used rubber and the bike clamps.
each muffler takes 4-6 db off so instead of 58 it would be 66-70db at ⅔ meter