on one of my past ute projects i completely cut the tub floor out, you will need a special drill bit for drilling all the spot welds out, not sure if you can do it with a normal bit.
Its a damn big job! but if you have the patience it would be worth it.
I also just measured up the floor area, went to a steel supplies place and got them to cut me some 16 guage sheet to size. Trim it up to suit and tack it in (was getting a mate to weld it in coz he is better at it then me lol)cut holes for wheel tubs and weld them in. Also from memory the standard fuel tank bracket is welded to the underside so you may need to reweld or relocate it.

As for being legal im not sure, you might be able to get away with it if it doesnt look too over the top, or just check with n engineer