a/c, non a/c cab difference

[Rob'68]

At some point before I bought my truck, a/c was added. It was originally a non-a/c. Holes were cut in the dash in the correct places for the vents. It's attached to the firewall with self-tapping screws. You can see where the original holes were. My question is this: What is the difference between a/c and non a/c firewall openings? I need to make sure everything works ok before the cold weather gets here. Thanks.