Well, I wish I could help better .. but are you saying you already have a SS X-Wing built but just want to take out the old cockpit and R2 and replace it with this one?
If so, I see some major problems (but again, I'm pretty new to all this)
1. If you have a V3, it means your canopy is fixed, so that means you can't fit anything through there (can't barely fit the pilot and I have taken off the extra lips). Anyway, V3 or not, it would already be glued and therefore, pretty hard to remove without breaking it .. So I'd think you'd have to cut off the canopy and get a new one. Then paint match and reinstall.
2. Due to the fact that the cockpit and furelage is tapered, I'm not sure how easy it is to insert the cockpit inside the closed fuselage .. but this is likely not an issue as I would think it's still possible to do.
3. To have access to your electronic or at least your battery, you're gonna have to drill a hole somewhere. In my case, I cut out a gap under the R2 strip to access my circuit and battery .. Were you thinking of going through the stand rod for power to your LEDs?
Hope this helps .. I'm sure I forgot a ton or details