If someone is going to take the time to steal your winch by unbolting it, they are cool enough and have enough time to load your truck on a trailer and tow it away whole and cut off the keyed bolts and every other part, I would think. Or they can just weld a bolt to the key and remove it that way, just like they can with wheel locks. Is winch theft a big problem here in the east? No offense, but there are lots of lighter parts that are easier to hawk than a winch.
Google comes up with 17,000 references to winch security bolts. Here
is a bolt cover and here
is a home-made anti-thief device, although my favorite method is described on this very web site: Zowie