Get a creeper and roll around underneath. If you have a fuel cell you will obviously have to be aware of that and depending on where you want the track you may have to drop one end of the tank to get your hands in..
There should be 5 steel cross members under the floor, they are in line with the tie downs you already have.
Here is an example of some added track.
There are 3 lengths going front to rear and two from side to side (one is hard to see and is by the ramp)
//heartlandowners.org/showthread.php/11365-And-so-it-begins?p=262148&highlight=track#post262148