LX monitor arms are an excellent choice for this monitor, and the 6-inch restriction in the back should not be a problem with the solutions outlined below.
You have a few options:
1) You could use four LX Wall Mount Arms
(45-243-026) and attach them to the wall behind the desk. Optionally, you could combine them with two Wall Tracks
(31-017-182) that would act as an interface between the wall and arms—with the tracks, you'd increase support, be able to forgo installing to studs in the wall, change the base-height of each arm fairly easily, and assure the bases are precisely aligned one above the other. A track would be installed on either side of the center monitors. Without the track, you'd just need to install each arm to suitable wall studs.
2) You could also go with desk mount arms: the most cost-effective solution would be two LX Dual Stacking Arms
(45-248-026). The only caveat with this solution would be that you'd need to lower your fixed-height, center monitors down a little. You'd have a clearance of maybe 3 inches or so between the bottom of the lower displays and your desktop. The pole isn't tall enough to get the monitors on top as high as your photo shows currently.
3) To get the screens higher (to their current position or even farther up), you could go with our LX Sit-Stand Desk Mount Arms
. You'd want to place two arms on a single base on either side of the center screens. The most cost-effective way of doing this would be purchase two Sit-Stand Desk Mount Arms (45-360-026) and then two Sit-Stand Wall Mount Arms
(45-353-026); add the two wall-mount arms to the desk-mount bases, leaving the wall-mount bases unused.
Hope this helps. Let me know if you have questions. Regardless, let us know what you decide to do.