MagicV8 is also right in the modders' opportunity though. Autopack can do it with addons, however if you don't like it, you can find a car mod and replace one of those "useless cars" yourself! It's as easy as firing up the VPE, changing the data in speed and handling, dimensions and name and you're good to go!
Myself I prefer to do it the Autopack way because those guys are working with maximum quality, however, as an example I love BMW's, and without the help of any modpack, just loose mods, I did this:
All with good handling, speed, and names and all done by myself.
My point is: just because Autopack doesn't add your dreamcars in slots you consider useless, doesn't mean it should stop you from filling up the useless slots.