If you go with a 4" or bigger exhaust system for your Excursion, you'll more than likely end up with the dreaded exhaust 'drone'. Search for it, lots of information regarding that issue. If you don't mind your factory 3.5" exhaust, most guys will recommend either deleting your muffler and going with a straight pipe or the Walker BTM muffler.
As far as tires go, I have the stock 265 Michelin LTX MS2's on my Excursion. They are great highway tires, wear really well, and get great feul economy. I can get 20MPG on the highway with my Excursion the way it sits right now with my current mods.
The next tire I will try whenever these ones wear out will be the Toyo Open Country H/T Tuff Duty. 285/75R16 would be the size I would go with.
OPEN COUNTRY H/T WITH TUFF DUTY | Toyo Tires
They look like a great highway tire, they would wear really well as well and are a load range E.