Hey, I have a '75 duster with a 400 big block in it (non original) with smog heads and an 8.2:1 compression ratio. I would like to raise thecompression ratio, possibly by swapping on a set of '67 915 magnum castings with a closed chamber design as opposed to the open chamber design of the later heads. I heard that with the lower compression pistons in the 400, however, that the combustion chamber design of these heads makes the combo prone to detonation. Would it be easier to just have the open chamber smog heads milled to decrease chamber volume? please help!