[lunarhighway]

as for the angle of the grill block, it doesn't really matter, it could even be flat, but what is important is the corner radius at the top (if you want to optimise) if you want to streamline a brick the first thing to do is round the corners. most gains will come from the fact air is forced around the car rather than trough the messy engine and out the bottom. alternatively you could make a grillblock that's adjustable... or removable off road.

a sloped fairing would work, as with the grond clearance on your car the air that goes underneath isn't as restricted... some larger dams "mud flap" style perhaps, in front of the tires could also give an improvement at speed...

perhaps look at the lates generations of suvs and picups for inspiration on airdam heights etc