When you're looking for a suitable gearbox, keep in mind that a multi-output gearbox can be used in place of a single or double output gearbox. For example: C400 G200 can be used for a C400. If you only need a single or double output gearbox, the Ctl+F find function is a very effective way of finding an appropriate gearbox that will satisfy your requirements, and the Wiki search function is also excellent.