What's the 5mm's you lose to your brakes? The thickness of the rotor at the hub?
Curious if you know if your brake kit has any built in offset beyond the 73+ brake track width. I know that AndyF's kits pushed the wheels out some extra amount at some point, probably when he started using his own hub. As I recall, this was so that late model 17" Mustang wheels were easier to use and put the wheel/tire in about the same place as it would be on a stock car. I know there was cross pollination between Andy and Cass, but I don't know if Cass's hubs are based on Andy's design. If they are, there might be some additional offset built into your brakes that negates some more of the positive offset on your wheels.