I've always preferred to do it this way. I like to use 2x3 and section it up into the floor, sit the bars onto - or into - it above the floor, tying the chassis, cage and floorpan all together. This as opposed to the outrigger being under the floor with the floor trimmed around the bars. The 2x3 can be mitered and made to fit nice and clean. Most people think this is too much work, but it makes the car safer and stiffer.