Where to find complete Heater Duct replacement

[SonomaComa]

I have a 1970 GMC C-20 without A/C. I have all new replacement parts for the heater system. Everything except the Duct work. I purchased them at Classic Industries. They offer 2 out of 3 parts for the Duct work. They have the Outer Heater Box from inside the Engine Compartment Part #17183 and the Air Selector Duct Assembly Part #G2776. But I can't find the piece that attaches from inside the cab to the Firewall. They only offer something called the Heater Box "S" Plenum (#AP7221) which looks nothing like what is in the truck. Listed here... http://www.classicindustries.com/sho...c-heater/?&p=9

I am looking for these 3 pieces. The very bottom piece separates into 2 pieces.