I am trying to locate a center underdash AC duct to replace the one in my 68 due to it being hacked up from the previous owner. What is the difference between the 68 and 69 duct? I have a part number for the 68 duct: 9796251......is that correct? It looks like the 69 duct is 9796231.... They look the same but I am assuming the 69 part won't fit on my 68.