You don't have to use Futaba brand extensions, Y harness, switch, ect... any brand will work.
Just buy it with either Futaba "J" connectors OR "U" connectors ("U" connector= Universal and will fit any brand radio)
I would think 8" will be long enough though and "Y" harness is the easiest setup and the way I set all of my planes up.
I like to put a piece of 3/8" shrink tubing over the connector on the extension so I don't have to worry about it coming loose in the wing.