Is there a mod that adds bare chassis/bodies?

I seem to remember there was a mod at some point that added bare bodies without other parts like engines or suspension so you can build a car from 'scratch', but I forgot which one it was.

 

I think you're referring to the "Rolling Shell Configuration Pack", it was marked as outdated due to the fact that it hasn't been updated in a year.
I've found that ArsPro's Stripped Configurations Pack is a great substitute, you just need to remove the seats and change the engine (or old paint on some configs).