The basic problem is, the ducts and box outlets are different diameters. The ducts themselves are 2.5 in diameter. One outlet on the heater box is 2 and the other is about 2.25. So, I ordered some 2 and 2.5 duct hose from Nostalgic Air Parts who sells hose by the foot along with their 2 to 2.5 adaptor. I attached a piece of 2.5 directly to one of the ducts in the dash. I then attached a short piece of 2.5 hose to the other duct, followed by the reducer, and then the length of 2 hose.
The nice thing is, the 2 hose fits on the heater box outlet snugly, and the 2.5 hose fits the slightly smaller outlet with enough interference that it would probably stay on by itself.