No spacers, they bolt right to the cross members below and to the tow hook bolts in the front. There is enough flex in the plates to make it work well.
The drawings are a start point. I would have the front brackets that tie into the tow hook threaded holes custom measured to get the angle right. Mine are a bit off... to lazy to make nice.
I used 1/4" aluminum.
I was leery about putting a skid on the gas tank as I didn't want to BOLT it on and wind up a Pinto. I would WELD a gas tank skid on the one thats there if you want that protection - if you are there, I would also have it extend over the cladding to the underside of hatch door. This is where you gouge your cladding coming off a steep rock...
Right now I'm playing with the idea of UHMW skids as some sort of gas tank skid/sliders...maybe bolt a plate of it under the rock sliders to "grease" the ride off a rock...
I hate touch up painting, so I paid through the nose to get good aluminum.