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You should be fine either way. 245 will be more flush on an 8.5" rim than a 255 would be. 255 would be only slightly wider. Not sure if the overall rolling diameter is of any importance but the overall rolling diameter of the 255 will be slightly higher that the 245, by roughly about a 1/2" or less.
The 255 isn't too wide, but IMO does look much bigger. I'd get the 245 but if you can try both tires on then you can decide which one is better for you.