You can't put an 18" tire on a 17" rim. That much surely won't fit. As for height, consider this...
The 265 70 17s that you have now measure out as follows...
The 265 is in millimeters, and it is the width of the tire, or about 10.4 inches wide
The 70 refers to height about the rim as a percentage of width. 70% * 265 = 185.5 mm. Double this value (top and bottom side of the rim), convert to inch, and add the rim height in inches to it, and you've got you're overall height. (185.5 * 2) = 371 mm, which is about 14.6", plus 17" rim height for a total of 31.6" tall. Just as a quick recap, your current tire/wheel is about 10.4" wide and 31.6" tall.
The tires you're looking at are about 10.8" wide and about 32" tall.
Hope this clarifies what you wanted to know.