Does anyone know of suppliers for replacement air dams for the late '70s full-size models? My '79's dam is drooping following some spirited, sustained high speed driving and I can't find a replacement from my usual suspects. I suppose the original had dry-rotted over the years.

As an aside, the car will do 85-90 mph all day long with no sweat.

It sounds like you still have your original, so you could make a new one from some new rubber material. When they get to this age the material is pretty brittle and they can tear out of the retainers easily.
Check your local hardware store for the material.