Doesn't mount anywhere underneath, just the 6 bolts on the side. The rearmost bolt is 4" long and is through a boxed section of the tub that braces the lower seatbelt mount. The forwardmost 2 bolts are in front of and behind the firewall, so you gain the rigidity of the firewall.
You can add metal underneath and run the skid under a body mount, that's pretty common and more slick that attaching it to the frame. I'd run it between the body mount and the tub if I thought it really needed it, but it doesn't.