Teddy is right. I salvaged mine out of a wrecking yard complete, as mine was built w/o a heater! Amazing to me even in Calif.
There is a rather thick rubber sleeve that connects to the fresh air fitting. I suppose you could use some wide black vinal tape or find an inner tube the right size to cut up. But Judging by the work of art done to restore that heater you're looking for show level orig. equip. I'll have to see that truck!
I go w/ whatever works and looks decent.....oh yeah and cheep!
Speaking of cheep, when I needed to get a new core I looked around and found an in stock (cheep) one for a later f100 that fit fine but had to mod the tubes a little to fit the firewall holes. Much less $ than a exact repl.
Hope the truck has the defroster connectors @ the dash, those were a bear to install.