I'm looking at an 00 excursion and it has some fairly worn offroad style tires on it, size 35x12.5 r18....
Am in a similar situation - looking to decide what I want to do with the tires and wheels on my 00 Ex. My 00 Ex came with 265X75 16s from the factory and depending on the suspension I understand I can go up to 35" without rubbing depending on the tire (I believe that is a 315). I do not want to have any rubbing so am looking at 295s maybe 305s. If you have 35s on there now with no rubbing you will easily be able to run the 265s or 285s.
Mine is the 7.3 and I should be running a tire with a Load Range E rating and would bet that you would not have any problems with a tire with that rating with the V10, maybe a bit of a stiffer ride, I would think that the V10 would recommend the E tire as well but I would definetly not go any lower than a D - I know a fella that went with Cs on a Suburban and they would "pop" on the highway.
Update - I have test mounted 285s and 295s on stock wheels and even the 285s rub on the leafs at full lock. If I want to put these tires on the 00 Ex. I am going to have to go with an aftermarket wheel.
Good luck with your hunt!