My 1971 Duster 6.1 Hemi build

Hey there...I remember you from when you had the other Duster.

It looks like you are running into the same thing I have (and have many other people with the 1st gen Borgeson box), of needing to shim it. I haven't done mine (it drives fine without shimming), but it will help with the clearance to the block and headers.