Hi Tristan,

   Something to consider: When you bend the top to the "ribs", the top will not reach all the way to the outer edge of the sides. To prevent this from happening, you may want to first shorten the end pieces by a little. Take off, maybe 1/8 inch from each end of the end pieces. This narrows the box a little.

To curve the top, I cut about a 1/4 inch off the top of the sides,(depending on how much of a curve you want) then lay the side piece over the end piece (bottoms flush) and make a mark on both ends of the end piece where the top of the side comes up. Now, remark that mark on the end pieces, about a 1/16 inch lower. This marks the bottom of the arch. Find and mark the top center of the end piece. Find something for a template to match the arch on the end piece, ie. a large plate, a dish pan,etc, or make one on power point, print it out and cut it out. The template should touch the marks at each edge of the end piece and just touch the top of the end piece at the center.
Using the template as a guide, draw the arch on the end piece on both sides. Make sure they are the same. This way if you have to flip the piece over in the process of cutting or trimming, you will still have your line to reference. If you want the bottom arched as well, it's pretty much the same process.

Once the sides and ends are cut, glue your side and ends together. Cut and install your end blocks, cut to match the arch.

 I use the strips that I cut from the sides to form the kurfing. I glue them to the inside of the side pieces at the top, overhanging the top of the side by about 1/2 of the thickness. (Clamping them is a booger.) I then plane the strip and side top, on an angle equal to the angle, where the end arch meets the side. 

Concerning the top: If your top is warped concave, it may give a lot of trouble bending it in the opposite direction. But with steam, persistence and luck you'll probably succeed. Though sometime you may have to give in and flip the top.. This would be a rare occasion. Most of the tops that I have installed, I didn't even need steam. It's good advice to inspect the top for cracks and repair them before attempting installation.

Dry fit the top to the ribs gently applying pressure to the sides. If it feels like it doesn't want to continue to bend, apply steam to it. Once you know that it will fit, glue an clamp it. Clamp the top at the center at the ends first. Cut your sound holes, install the bass bar, mortice and install the neck and back and you're ready to go.

I hope this makes sense. Of course, all of this is dependent on the size of the box, what kind of box you're using and what you want to accomplish. If you're looking to build an acoustic instrument, the above is relevant, but if you're going for electric, some of it may not apply. In any case, this is what I've figured out and I hope it helps. Thanks for the interest. Craig