The only way to increase the ride height with the stock shocks is to unscrew the shock end. You know, the peice that screws onto the end of the shock shaft. You might also try some of the alternate shock ends for the super nitro or other kits that are longer. I do know that some of the racers in the box stock class were cutting down the shock shafts to lower the cars for better handeling. You can also add an internal spacer inside the shock to limit the droop or down travel of the arms.

Just reread your post. If you have already lengthened the shocks, try the longer ends. Have you added more preload spacers too?