the strength of a 2x? is vertically not horizontally. I personally would have put them together a bit different because I normally don't aim for "very little shot" when I'm under a 3,500 lb car. I normally go for "no way in hell". :)
But I'm sure those will work out just fine. I personally just use four 3 ton jack stands whenever I needed my Z that high. Never had a problem or felt in danger.