Two things. What year are the cars? The Mustang has lots of years with five or six generations now, the Viper has lots of years with two or three generations now, and the Audi A8 although is relatively new and doesn't have many years, has like 10 models available. Two, I can tell you nobody will sit and make a content pack for you. People may sneak the cars in their packs if you ask politely on their thread with a car or two at a time, but beyond that, you're not gonna go far.
On that, I've been tossing around the idea of making a Koenigsegg Agera R, an old Dodge Challenger, and a new Viper. If someone doesn't before me, I probably will throw those in my pack. However, I'm too busy trying to make my damned Eclipse actually show my Plymouth Barracuda so I can see it and make sure it works. Mind you, when you're getting 100 errors, that doesn't help, but you know.
I'll be thanked for killing a monster while you're burning in hell.