Can I fit a 265 75 R16 tire on my 2005 express 3500?

 

Your OEM size is a 245, so, you'll be increasing the tread width by about 1/2", which shouldn't cause any rubbing given the room most Express' have. So, as long as you keep with a "E" Load Rating, you should be ok.

Keep in mind though...
When you increase the width of the Tire, and not the Rim as well, it forces the beads inward compared to the Tread. The Sidewalls function best when they are running vertically. ie: Tread width and Rim width should be the same for best performance and loading ability.