you only have 3.8 cf before displacement meaning your probably only have 1.6cf per sub after displacement this would work good for 2 ten inch but not 2 twelves IMO
if u want a burp box watch how this guy gains 10 db from trial and error u might be closer to 147 than u think http://www.stevemeadedesigns.com/board/topic/98191-spl-gains-updates-on-page-16-slot-flares/