Custom Races and BOP

[Talon16]

So I started setting up some Custom races around N24, but BOP is applied to the cars. I did a Gr4 and a Gr3 and wasn't really paying attention until I wanted to use my Ford GT Race car. The BOP won't let me add ballast to the front of the car to help stabilize it. I can add down force and such, but not much else.

What am I missing? Is there a way to do Custom races against the AI without BOP activated?