Thanks guys!
I have a stock 2008 12 Passenger 5.4 V8 and its being converted next month. I was considering waiting to change my tires/wheels till next year as the dealer had just put 4 new nice all weather tires on and it seems to be a waste to just trash them.
And yes I had talked to Advance 4x4 who said that the 285/75/16 is not an issue and a standard size for many of the vans they convert. I'm just a complete novice when it comes to car and it never hurts to get another opinion, especially as I may wait tlll spring to get the wheels/tires replaced at Canadian Tire and they aren't the biggest Van conversion experts