Make the bumper hollow, with the winch sitting inside it at frame rails level with the feet forward. Most winches get installed feet-down, when you get to pulling that puts a big axial load on the rear bolts, a bending moment at the front bolts, and shear on all four. Those are relatively small bolts threaded into aluminum castings... Bolting the winch feet-forward takes care of all that, and the load from the pull puts the winch legs in compression which is their strongest position. We have Ramsey worm-drive winches on the bigger trucks (10k, 12k, and forgot what the big monster was on the MDT), they all get installed with bolts from the front and the back, none from the bottom - this is how they re designed, as again this is the strongest attachment method possible.
If you have concerns about the factory mounting plates, just add some to your bumper that will reach further back on the frame rails. Two 1/2" grade-8 bolts per side should be plenty strong, as like I mentioned before in the typical feet-down installation the winch itself is held to the bumper by four much smaller bolts that have to handle the same loads and then some.