Radically altered vehicle?

Hi all
Hoping someone could help me in regards to understanding the law regarding altering a vehicle...
I plan to remove the rear bed of a pickup and replace it with a wider custom bed. This from my understanding I can go out 305mm to either side?
If I do this (and all otherwise is completely stock) do I need sva and a change of weight on the logbook?
The dvla guidance is clear as mud to me!