I gave the measurements to a friend in the HVAC business and he's getting a quote on parts. Parts should run somewhere around $200 + labor.
Most of the visible duct work here would be cut out. Then 3 pieces would be installed. The bottom piece would be a 7 inch offset to the rear side of the cavity. The top piece would be a square corner that contains deflectors. These would be joined with a 14 x 8 inch sheet metal column.
Flexable duct is not recommended in the main duct work coming out of the furnace.