The only thing I might further suggest is that, if you want to ensure that the layers of your crib stack stay in position, you could either go with a vertical slat, a length of 2x4 and screw one of them to each end of the stack as a rib or brace, or you can prestack the number of layers of cribbing you plan to use and then clamp the stack together so that you use a 1/2 inch drill motor and a 1/2" dia auger bit to drill down through the corners where the rails and crosses overlap. After that you slip a length of 1/2 rebar into the set of holes and the whole stack will stay as laid. The alt to that is to go with all thread, get 4 lengths of 3' x1/2" coarse thread, cut them to length and use nuts and flats to do the securing.
For myself, I prefer the 2 x 4 vertical braces because you can locate them either on the ends of the cribs or on the sides. If you go with the end, leave a little extra and you can build a bump stop so that even if you are on a slight slope, the wheels can't roll off the stack. Had that try to happen to me in the front drive which is sloped to drain water. Added two vertical ribs on the low end to lock the layers together and then slipped in a 2 x 6 (2 x 8) as a chock block for the wheel to keep it in place. A few more deck screws to hold it all together and I was able to slide under the van to replace the rear driveshaft cross bearing. If you are really worried about shift or slip, then put one layer of cribbing under each of the front wheels and run a stringer from the front layer to the rear to lock them together.
One other note is that you can actually use the cribs to locate and raise a truck rear end assembly if you just keep stacking layer by layer, one side at a time, using a floor jack to get the room to slip the next layer in. Actually watched a guy to just exactly that with his lifted four by. Got the truck up on jack stands under the frame, pulled the wheels and put in the cribbing to support the rear end, dropped the suspension out, and then layer by layer he removed the cribbing until the rear end was on the ground. Drug the corpse off to the side, went and got another rear end from the salvage yard, and used the cribbing to lift that rear end until it was high enough to rehang the suspension. Put it all back together and drove off. Did it all there out on the front lawn of his rental. Didn't even have a shade tree, just a few stumpy pine bushes.
Nick