Well, when I do something like that (buy decals designed for another kit but I have a better kit on hand), I photocopy the decal sheet (ensuring the size is exactly the same) which allows me to cut it out, wet to soften the print paper, and 'mock' apply to the kit! If really off, I take another photocopy which is then glued to masking tape to create a... mask! The gaps are then filled with masking tape strips and needed & matched colors are sprayed on.
Luckily, you are to use it on a larger scale kit! The smaller the kit, the higher the difficulty factor. Since the RAF 112th Squadron sharkmouth were in three simple colors (red, black, and white), you can also apply the decals and paint over the gaps (if not for competition) or paint over the gaps and decals to ensure a uniform color. Lat option is to use the same red/black/white from other markings on the decal sheet to cover gaps as this ensures the colors match.
I hope this helps!
