Nice job on your box. I looked at the MB shallows as well, but found a deal on the Phoenix Gold Comp 10s. I'm running 1100 watts to them at 2ohm and they hit hard but can also handle fast tight bass. I designed the box to where I could easily reach the rear seat release, I'm just waiting on my weather to get a bit better to finish it all up.
8's will work, but you'll loose some low end if that's what your looking for. I think I'd do one really good 10 before I used any pair of 8s. I just haven't found an 8' that provided a noticeable gain.
If your not running the rear seat then you have tons of options for building a box that doesn't waste space. If I wasn't running the rear, I'd build a larger box that had storage (perhaps a slide out drawer) in the back section for tools, straps, etc. and then down firing subs in the front. You could build it 3' or so wide and as deep as you wanted from the tailgate. You'd only need the first 12-14"s in the front for subs as big as 12s and wouldn't need to build the box any more than 10" high(more if you wanted). I'd use 3/4" birch plywood rather than MDF to keep the weight down.
"When Bruce Banner gets mad, he turns into the Hulk. When the Hulk gets mad, he turns into Chuck Norris."