Over $6000 to install TTI headers on 66 Barracuda

If no one else has mentioned it, I'd consider paying an attorney $100 to write a letter explaining that the charges are "well in excess" of normal, and ask that a standard diagnostic fee (or something similar) be charged in exchange for the release of the car.

Also, you could suggest a mediator or arbitrator.

Either way, you're likely to get out for a LOT less than 6 grand with no work being done.