Pitching isn’t so much about the shoulder as it is about the hips and thoracic spine. Being able to go through extension and a huge amount of rotation in the thoracic will help to load the shoulder for throwing the ball. This means training the core in a different way than sit-ups. You want to try and create an elastic recoiling through the front core, meaning to stretch the core in a way that represents throwing, and use that ‘stretch’ as a slingshot (similar to releasing a rubber band) to throw the ball effectively. Standing with rotation exercises would be a good idea here.
For the lower body, getting a lot of rotation through the hips would be important. Rotational lunges both away from standing leg and crossing standing leg are needed for throwing. Then progressing these movements to single leg balance will help with the deceleration of the throw (where the pitcher tends to hurt the shoulder).