Using this link: Tire Size Calculator - Compare Tire Sizes
Your speedometer will be off by about 4%. It will read 4% slower because you are going to a larger diameter tire. When your speedometer says 50mph, you'll really be traveling 52mph. Not that much of an issue and will not have any adverse effects on transmission or anything else. In fact Ford speedometers tend to read a little on the high side compared to actual speed anyway.
Also, 255/70R16 is the factory size on '03+ 4x4's with 16" rims. Prior to that, they were 245/75R16 (a little skinnier but the same overall height). You'll be fine. SWAP AWAY!!