I'm not convinced that Aluminum is a good choice for a skid plate. Aluminum is soft and the surface has a tendency to gouge and tear. I think it would be fine for skidding over snow, but not very good for skidding over rocks.
I would recommend the 6,000 series aluminum for corrosion resistance in a salt spray environment. 6061 would be the best choice due to its yield strength of 40,000 psi and an ultimate strength of 45,000 psi. 6063 would be a bad choice since it has a yield strength of only 21,000 psi (but it looks nice).
You will need galvanic isolation to prevent corrosion where the aluminum contacts the steel frame.
Another factor to consider is the modulus of elasticity of aluminum is 1/3rd that of steel, so for the same cross section the aluminum will bend 3 times as much under a load. This can be both good and bad depending on your design.
I have seen H3's and an H3T (mine) high centered on their UCP with almost no weight on the wheels, so I would design the UCP to carry the full weight of the vehicle without yielding. My UCP is developing a curvature which I'll have to rectify someday.
If you want to do a lattice type design like the OEM UCP, which I think would be better for hot operating conditions. I would recommend ASTM A513 also know as DOM (drawn over mandrel) steel. DOM is only available in round tube, but it has a yield strength of 72,000 psi and an ultimate strength of 87,000 psi.