Displayed text when in-stock (Make Dropdown in admin)

[airbag]

Hello, I am wanting to make the "Displayed text when in-stock:" option in the admin a dropdown box with 3 options.

in stock
within 48hrs
within 2 days

This way we can keep the same language throughout the store.

Any ideas how to do. I am happy with it displaying on the frontend, but just require the admin side to be a drop down options instead of the text field it currently is.

Thanks for any help.