Hi. I was reading a book on insulating, etc., which advised to seal up any leaks in the ducting. So, I found a good size gap in a junction where an air return duct joined the main air return duct. All the branch ducts for the air returns are just a piece of sheet metal attached to two floor joists. So anyway, after looking at this branch duct I decided to completely remove the peice of sheet metal because it is rusted and on the inside was 50 years of accumulated pet hair and stuff. I was thinking that I would put in a real duct in its place, and I was thinking of using either 6 inch round or 8 inch round ducting. The opening from the baseboard register into the basement is 12" x 4". I was able to find a 90 degree converter for the 6" and the 7" round ducts to the rectangular register, but not one for 8" round yet. As for the junction from the branch to the main, it is just a large hole cut on top of the main air return duct where the branch ends right over it.The length of the branch duct that I want to replace is 10 feet, and it is the closest one of the return ducts to the furnace. Does this seem like a good idea? Which size ducting seems most appropriate? Is it worth trying to make an adaptor out of sheet metal in order to use the 8" round? I would appreciate any thoughts or comments. Thanks.