This is what I used, it has amazing instructions and so I far I managed to add about 60 vehicles to my server.
Some vehicle packs wont work no matter how correctly you followed the instructions, and not all vehicles in a pack will work, the best thing to do, is to name each stream folder exactly the same as the model files.
Add a test car, just 1 car model, maybe 2 encase that model is just defective, then see if it loads, if it can load at least 1, then its all set. Then, add only the cars you want to use, and load up the server, use a trainer to spawn the cars one at a time and see which ones load, don’t load, or strait up crash the server.
You can delete model folders out of the stream folder while the server is running, so you can weed them out and keep up. After that, if you are skilled enough (trust me, this isn’t that hard, but it might be too confusing to some people) go into the vehicle meta files and try to erase the lines that pertain to the cars that aren’t there anymore, mostly out of the handling.meta and vehicles.meta. If you can’t figure it out, no worries, its just kind of tacky to leave in entries that can’t be used, no big deal.
Don’t forget, every time you delete a model or make an edit, before you restart your server, clear that resource from the cache.
Good luck, I hope my little tutorial will help.