Losing weight should not have actually reduced the size of your pectoral or biceps muscles. Perhaps they they were not so visible when there was additional fat present. Strength is completely different and may well be a result of less developed muscles. When a chest muscle is "not there" it is ...
Your assumption is correct in that your new implants would need to be custom made. They key to your new custom pectoral implants is that the base width of them should be the same but the projection on them needs to increase. By so doing your chest will appear fuller/bigger but not wider.
Pectoral augmentation will improve the look of your chest by making it appear larger and therefore decrease the gap. You wouldn't be able to re position the nipple without creating a scar and I dont think its worth it in your case. However adding a pec implant would look great.