Basically you can choose to increase your power, or shot count. On my Avenger I saw more power after adding a plenum extension without making any other adjustments. I could chose to lower my tune settings to increase shot count some, but I want all the fps. And I have a 300cc bottle conversion so shot count is like one million. You probably dont actually have to make any adjustments to answer your question, but can and likely will want to.
For the stock though, you will obviously need to remove some material to make room for the plenum extension. I have a wood stock so I used a hole saw, file, and dremel.