It's actually even more complicated than that!
Your truck came with P metric tires - on 7" wheels. All the sizes you mentioned are LT's.
If you are planning on using your stock wheels, I'm afraid the size you have - P265/75R16 - is the largest that will fit.
If however you are planning on changing wheels, here's a calculator to help you with the diameters and stuff.
http://www.1010tires.com/TireSizeCalculator.asp
Hope this helps.