Tubbing the rear end..

I'm sitting at work doing very little and I have had this great idea to tub the back end. I'm replacing the 10 bolt with a new 9'' so I could have that built to suit. I have the bare cab sitting in the garage.

Few questions..

Is it legal? I was thinking 2'' either side
What other factors do I have to include?

My other thoughts were to recess a fuel cell/drop tank. So both these could happen at the same time.

Thanks Jenko