Boring .030" will yield only about 3-4 cubic inches in increased displacement. Milling the heads reduces combustion chamber volume and increases compression by a point or more depending on other engine factors. Milling too much is just plain bad as you can't really get it back once it's gone and you can easily wind up with an engine that won't even run on premium gas. You may also need to make adjustments to pushrod length. Stroking is a more effective way to increase displacement. Adding nearly 1/2" stroke to a 5.9 yields just over 40 cubic inches in increased displacement.