P and P internal roll cage

I have bought a protection and performance 6point full internal roll cage off ebay. Just wondered if anybody had fitted one before and how they had mounted the rear floor plates to the chassis? I was definitley going to put steel box section between the top of the chassis and the floor but would it be better to weld the box to the chassis or use big hi-tensile bolts and plates underneath the chassis legs to clamp the lot together?
Thanks in advance, Harry