Wow the P-40 is really looking GOOD !
As far as the decals go, Yellow Wings has a decal sheet available that has the numbers and letters you need available in white. They also make the same set in black if for some reason later on you may decide to build something requiring black letters.
The part number for this decal sheet with white letters is 48-024, if you're building a 1/48 scale F4F.
It's labelled as:
48-024
USN 1932-1942
Standard 12"
Numbers & Squadron
Designators WHITE
The same thing goes for the black letter and numbers set, but the part number is 48-023.
I'm sure they also offer this in other scales if you are building a kit in 1/32 or even 1/72. Hope this helps...