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I would recommend a 4 point mounting through the body to the frame. Two box tube cross members across the frame separated by 4 feet. Then run 2 more box tubes longitudinal, front to back on top of the new cross members and spread at a distance appropriate to pickup the winch mounts. Due to the capacity of the winch and its ability to lift the truck or pick up the front end when winching it would be important to spread out the anchor system over a larger area, thus a 4 point mounting with good space between the mount points. Either professionally weld the box tube cross members to the top of the frame rails or study how gooseneck and 5th wheel hitch frames bolt to the vehicle frame and copy one of the better designs. The box tube should be at least 1/4 wall and 4x4, 5x2, 5x3 would probably be okay. The longitudinal members should be heavier 3/8 wall. A 1/2 inch plate under the winch would be good for mounting and load distribution. If weight is an issue then step up to larger materials near and under the winch and thinner materials where attachments to the vehicle frame are located. A winch of that size will put a lot of load on the winch frame at the location where it is bolted up.
