There is no specific pathname you need to change when saving a an existing template in the Custom area, you should need only name your custom version to be able to save it.
Unfortunately there are currently known issues with the supersize feature when customizing an existing template and with the Slideshow options for image layout. There isn't an issue with what you have attempted, but an issue with these features when customizing templates. Our engineers are aware of these issues and are seeking solutions.
I will update this thread as soon as more information on these issues becomes available.
Apologies for any inconvenience this may cause.