Check out this tire and wheel calculator:
http://www.rimsntires.com/specs.jsp
What's nice is that if you put in tire sizes, it recommends rim widths for that size of tire. You can also look at backspace and offset too. A 265 on a 10" rim is on the narrow side.
It's easy to figure a tires nominal width by its size. 265mm divided by 25.4mm/inch = size in inches. So, 265=10.433". However, this is not the actual overall width. Rim width and aspect ratio affect the actual. If you look on Nitto's site, I think they list actual cross section width when mounted on a particular rim. As the rim gets wider, so will the actual width (not one to one though).
Also, a 10" rim is actually wider than 10". The width is on the inside bead-to-bead dimension. So, I can't see a 10" rim with any size tire working on a 65 without some wheel tub mods.