65 barracuda 273 commando

Don't go too big or you will suffer from poor driveability and mileage. I think 600 cfm max and 450 or even 370 would be better. You can find plastic base adapters on ebay, but I don't think any way to adapt a bigger horn (new carb) to the smaller hole in your factory air cleaner, so plan on a new air cleaner. I haven't dealt with it, but recall reading that the 4 bbl intake on your car has a different (smaller) bolt pattern than either the later square-bore (most common) or spread-bore pattern, so you might need a base adapter plate.