Maybe I'm confused about this. I thought the Ultimarc 360 could be anything 2/4/8 way depending upon how it was programmed. Why do you need a restrictor plate?
Encryptor
You don't need one at all. It just limits the throw of the joystick to give you a bit more of an authentic feel depending on what you're after.
For example, you're playing Ms. Pacman, a 4-direction game. Even though you program the stick for 4-way play, you can still move the stick in any direction you want. If you install the 4-way plate and you'll only be able to move it in 4 directions.
Like DaveMMR said, they're not easily switched once installed, so you'll have to generally make a decision and stick to it.