235/40 will be too tal in the front. Go with either 235/35 or 245/35 in the front. As many have said the factory uses 235/35 and 295/30 for the 997. The 996 was not designed with 19" and the tire height of the 295/30 is about an inch taller then your 285/30/18. The 275/30/19 is closer to the exact size of your OE 285. I do run a 315/25/19, but it won't fit on a 10" wheel. A 305/30 is way too tall and 305/25 will be kind of short. Your best bets are 295 or 275 in 30 series.