Lead was added to the gas back then and used as a lubricant. It prevented wear on the valve seats. Since a '67 or '69 year model car is over 35 years old one of two things have happened. Either the engine has since had a valve job and hopefully the valve seats were replaced with new hardened items, hence the lead in the gasoline is not necessary. Or, the engine is still on the original heads and the valve seats have long since seen their lifetiime of wear (unless the car has extremely low documented miles. But it is irrelevent anyway as most people agree that the new formulations of gasoline have lubricants in them which perform as well if not better than lead does.
At any rate, don't worry about not being able to use leaded gas. Buy the car you like and drive it, when you start seeing indications that the engine needs work, then just fix it, that's why we buy these old tanks anyway right?
