The 245/40/20 tire will be 1.4inches taller than the stock tire. 245/35/20 is 0.5inches taller than stock. You're looking at about an inch difference. To me, the 40 sidewall seems too large. And according to the tire size calc you're speedometer will read 5.6% too slow. In other words, when you're driving 60mph you're really 63.3
I'm running 35 series sidewall on a 20" rim. I'd go with those.
Chad