Different websites different figures if you go to www.wheelsizecalculator.com and put in 350Z front it says my fronts should be 245 35 20 not the 30s I have
No, the 30's you have are the right size, 35's will give you a tyre that has a diameter of almost 2 cm larger.
I think he car LOOKS higher, but thats just because of the larger wheels, I'm pretty sure it's within a mm or 2 of stock height.
245/35/19 is stock size for 19's, your 245/40/18's are smaller than stock, so it makes sense that the car would ground out more often.