those various layers aren't (vertically) bolted together that you consider adding some external strapping to keep them aligned/well stacked once the desired height is reached.
FYI - I used 2x4 cribbing for my tires and the lumber did slip a little as the car was set down. Ended up using some glued sandpaper between the layers to increase surface friction. If your garage floor isn't relatively flat, height on the stack just makes matters worse - so pay attention to that too.