1968 Barracuda Dash Speaker Access, Size & Replacement

What are you planning to do for a radio? Are you just using the stock AM unit? Are you using an aftermarket stereo?
Depending on which direction you go, you'll want to make the appropriate choice based on the radio's ability and recommended impedance. If using an aftermarket stereo, by all means install the dual (probably 4 ohm) speakers. If staying with the factory unit, a mono speaker rated at 8 ohms is what you need. Or at least there's no reason to get anything better.