Wells 401 v3 External/Barrel Mod

Hello I brought a Wells V3 as my first blaster and replaced the insides with a metal box, replaced the buffer tube and stock and brought a 10" Handguard as I was told it will swap on easy by removing the connector at the front, but my main question is about barrel sizes.

As I want to put a new metal barrel with the front on I brought a 35cm Fishbone, do I need to replace the inner barrel with the same length or is the stock one good enough?