With 2 pounds of chocolate and 2 pounds of crystal, you might have plenty of body without having to mash so high. Put the recipe into hopville to see what the estimated FG will be?
I've never added brown sugar directly to a beer. It will contribute some molasses flavor I think. With a beer this big you might want to add the sugar to the fermentor around day 3 of fermentation. My only experience with sugars is making invert sugar for my Belgian beers. I love barrel aged beers, good luck with this one!