Here is some more info on how I mounted my jammer heads:
I got the jammer heads exactly perpendicular to the ground by using the level and marking a black line on the inside of the grill with a black Sharpie to which to align the head. You can see the lines in one of the photos. I then took the grill out and put the head along the line to see where and how to fit it, the holes for the brackets and how to bend the brackets ( I used stiff cardboard templates of the brackets at first and then aluminum which is easier than steel to bend and does not rust.
My grill slats are perpendicular to the ground but if you use one mounting hole in the underside of the grill (not the slat) and one on the slat then in combination with how and where you bend your mounting bracket you should be able to get yours perpendicular without using shims. But if the head is not flush with the slat, aesthetically you may want some kind of filler.
I personally confirmed with the LI manufacturer and the US rep that LIs can be mounted vertical or horizontal. The key things are to not block the line of sight, and have the front surface perpendicular to the ground on both axises and to have the heads as close as possible to all highly reflective parts (license plates, headlights, whatever).
Mounting them entirely behind the grill is great aesthetically and for security, but it will block some line of sight for side shots and perhaps some from overpasses as well.