Your compression will be a little low with 6X heads on a 400. You might consider putting a stroker crank with a 4.21 stroke that would get you in the 9.0 to 9.5 range depending on the bore size you plane to go with. That extra compression will wake it up not to mention the extra cubes you will gain. Butler Performance has a stroker rotating assembly that should do fine for you.
This is what you need, they may have a more economical 4.21 stroker kit with different pistons and rods
https://butlerperformance.com/i-2445...tegory:1234863