Here is a nice car picking utility to solve this very issue.
I always use it when I race in single player to pick exactly which cars I want to race with.
This also allows you to use any of the new add on cars with any car you want
For instance you need to pick the Cortina if you want the add on Toyota to show up in single player, If you want no Cortina's to show up in your race you can just turn them all off.
One thing you need to be aware of is that if you have used it to “turn off a car” that car will also not be available for on line use.
So if you join a server online and someone is using that car your game will not be able to load it and you will get that looping load screen thing. So I reset all the cars I’m likely to see online back on. For when I feel like getting hit from the back or blocked and want to do some pickup racing LOL.
This also works both ways
So if you are hosting an online game and say you want to let the 289 Cobra run, but you don’t want a bunch of jerks in 427 Cobras showing up in your race you can set them to off and anyone trying to join with a 427 will get the car not on server massage.
This is a bit clumsy but it will work
Here is the link.