Welcome Bprh, your van is a unicorn. I'm curious what the interior stand up height is now with the dropped floor?
As for tires, you've gotten some solid advice but I wonder how the lifted body plays a part in this? Surely 265's won't rub but I wonder if you are pretty close to getting away with 285's (if you want to try) since the body is already lifted. I've had both on a Ford but can't remember, the 285's were on a 4" lifted Quigley. I do remember the pinch weld/rear of the front fender can be a problem on Fords but it's easy to modify. Modify, in this case, means hack it away without mercy until you hit something important.