The gasket between the Upper Body Rear Panel and the Quarter Panel was a material that looks much like modern day Friction Tape. I have never found Ford's part number for it however I am certain there was one. Today when we have a body panel apart such as this, we use Seam Sealer in its place.
Brent,
Do you have a preferred brand of Seam Sealer that works for you?
A big THANKS for posting pictures of your restorations. I have used them multiple times to see if a newbie like myself is on the right track in my restorations.