Definitely check with twobits for their recommendation.
According their webiste, the art is on vinyl. If that is the case, then job should be fairly easy. Archer MacLean also uses vinyl for his excellent repro side art work. Vinyl is much more forgiving on the underlying surface condition than other side art. You should just need to get the current sides clean and smooth. They do not need to be glass smooth as the vinyl will hide imperfections that other, thinner, art will actually highlight.
If it is not on vinyl, then I recommend you do some further research - especially for full side pieces. There was a youtube video on applying sideart not too long ago...
You've paid a fair amount of money for that art, so take you time and plan how you want to do it. A second person to help is always a good idea when pieces are that large. For really high-value, rare, sideart applications, I've actually used a local auto tinter to help. He knows his stuff and the cost is minor relative to the value of the art.
Oh, dry applications are almost always best if you have the skill and time to do them. I've never heard of doing vinyl via the wet method.