265/35/20 is gonna be very wide for the LS. What size are your wheels? 20x8.5? if so then you can maximum put 255s on it, but usually most LSs with 20s rub with 255/35/20. Of course it depends on the offset but that's the usual. If your offset is 48 or 50 or so you probably would be ok with 255s, but if your wheels have offset of 40 which most LS 20s do, you have to stick to 245s. If you desperately want wider tires, try 255/30/20, although the sidewall is very small, but I don' think that they will rub much.
If you got 20x9 you can fit 265/30/20s on the wheels, but they will be just ridiculeslly wide for the car.