Not all CCs are candidates for this upgrade. Remember: You are modifying the unit beyond its designers' original goals, so your mileage may vary. At minimum, you may have to abandon the idea of obtaining full screen size. First adjust the PB control until you have the least pincushion (not pushpin) distortion. Then reduce the width until the distortion is tolerable. The result is the best that you can get with the controls that are available. If the result is still not to your liking, you'll have to revert to the unmodified state.
The pincushion circuitry can probably be modified to provide larger correction range, but without a schematic, it's a bigger job than most will want to tackle. Years ago, I promised Stuart Bell that I'd provide him with a schematic. Naturally, I still haven't gotten a round tuit (although I desperately wanted one for Christmas), so that project remains to be done (along with countless others)...