in answer to both of you, sealed boxes and generally fairly forgiving so a decrese in box volume by such a small amount wont really affect anything,you probly wont be able to hear the difference at all anyway. so you dont really have to adjust any dimentions.
yes thos type s's go pritty well in a sealed box too,for the money there an excelent sub.
Jack
EDIT are you planing to just built a whole new box or modify the alpine one? is the alpine one a sealed box or ported?
if its sealed box decresing the volume will just raise the tuning frequency a small amount which you probly wont be able to hear the difference.
id recomend building it equal in volume as the original one just to keep things the same,if your already modifying part of it you may as well do it all properly.