What are the disadvantages of putting a lower octane in my 65 barracuda? I have been using premium, but with rising gas prices I was thinking of using regular.

In my opinion you are better off with regular if you have a stock SL6. The ADVANTAGES are better gas mileage and more power. The only time high octane is needed is to prevent detonation, while allowing you to run an acceptable ignition advance, if you have high cylinder pressure (forced induction or high compression).
Bob D