I believe the 60/70's muscle car values are in a bubble. 8 / 10 years ago a few retiring baby boomers wanted to relive their youth, or finally get at long last that car of their dreams. Demand ramped up and values quickly followed. this snowballed when shops and responders saw the market, and opportunity.
This bubble is deflating.
Now, that said, truly unique cars, cars that are generationally special, will continue to hold value and likely rise.
All them mustangs, GTOs, firebirds, cudas, etc will be worth more in scrap then in the used market. cars like the original Yanko camaro, or Shelby cobra will continue to rise. Special editions will drop.
Exotics will rise, however at a slow pace. the europa i think falls here. It will likely stay at the 10k to 20k range for next good number of years.
As the baby boomers die out, the gen Y that are replacing them will have no affinity for those 60/70 cars. they will be looking at much new cars, NSX, Vipers, Ferrari's etc.