Because Vintage Air sells the mock up simply based on Gen 2, Gen 4 etc non vehicle model specific with the out put duct configuration of your choice. You then have to order the brackets for your particular application. The brackets are based on holes being in your stock firewall that are now gone with the custom firewall unit and look like hell anyhow as they protrude all the way through. So you can either weld bolts (elevator bolts are a common choice) or use a different style bracket like Hotrod Dynamics sells.