Question
I didnt understand this-if you are set on your weighted situps then you should focus on maintaing your bodies natural curves to distribute the forces sustaind during exercise. Your area (thoracics) that is in the mid back, between you shoulder blades should be ROUNDED IN THE BACK? (convex in the back).  

Answer
Jeff,
Sorry about the delay in answering, it appears the servers were down over the weekend.  Just a breif note about the spine.  You naturally have four curves when you look at the spine from the side.  The neck (or cervical) has a lordotic curve.  This means that the curve is toward the front and leaves it rounded in the front and hollowed somewhat in the back of the neck.  The same lordotic curve is found in the lower back (Lumbar area).  The mid back (reffered to as your thoracics is the opposite of these two and is rounded in the back.  These curves are great in design because they allow forces to be distributed over the spine and can sustain a much heavier load before failure occurs.  
The idea is to maintaing the curves as much as you can during your exercises.  Try to keep the weight during a weighted exercise close to your core and not too high on your chest.  Inhale down and exhale up.  Before begin to sit-up flex the abdominal and maintain this control throught out the exercise (even on the way down).  This will ensure your muscles are taking up as much slack as possible.  When coming back down toward the ground do so in a slower manner to maintain control and prevent injury.  You may not get as many reps in but you will get a great work out.
Hope this helps.
Dr. Wiggins