defrost ducts have seals?
 

Was wondering if the defrost duct under the dash is supposed to have a seal at the bottom and the top? It has a gap at each end and I see no evidence that there should be any seals. Seems like it would be more effective if it did.

No seal that I have ever seen, but on that duct, and on any of the a/c ducts, I think that a cursory seal would improve airflow performance. Maybe a thin compressible rubber gasket along the outer edge of the male ends, and a similar piece on the inside of the female ends.
Although not factory, I think everyone who has factory air in their cars could benefit from this.

No seals used originally. I've had a bunch of early GTO's and have torn apart many more at junkyards and none have had seals. Some high density foam rubber could easily be cut and fitted in place though. I agree it would be more efficient.