your best bet is to have a look in the back of an FC truck take a picture and take the truck to a steel fabricator, it's not a particularly difficult job. The higher up you can mount the winch the better. When you choose winch don't pick a big one because their is tendancy to try and pull the truck out if you get stuck and the difference in frame and mount point strength required for pull a deer up and debogging a truck is huge just ask the lads round here who have had to go to the boss in the morning.
Having converted several artic trailers for transporting xl size mobile homes which all had 2 large 24v winches' front and back to pull the on and off. Most of them were fitted on 1" thick plate using 10.9 nm bolts.
However you do it don't use cheap soft bolts, 8.8nm at the very least.
I've got cargo tracking fitted around the loadbed and on top of the drawers, and it provides an adequate mounting point for pulling the up to 200kg weight up the ramp and into the back. I've now upgraded from the plaster's bath and el cheapo motorcycle loading ramp to a specific deer/game sledge (Frankonia 60 euros) and a more sustantial and stable (if pricey!) MultiRamp from Roadranger that clears the lowered tailgate.