One of the small reasons the newer trucks are SO much better. The A1R uses the plexi-glass defrost channel. Visibility was improved since accidents due to poor visibility and lack of a ground guide are the some of the most common and most unfortunate accidents that occur in the military and they make efforts to mitigate accidents. This was one of those changes. But the list of "enhancements" made to the A1 and later the A1R is LONG and I can't even begin to list them. The gauge cluster got a sun visor on the A1R that is ridiculously expensive for no reason I can determine - going to have them reproduced since every time I see one it goes for hundreds of $$ and the military cost is also extremely rude for what amounts to a bit of sheet metal and a handful of bends. The rarity is the issue since only the A1R and A1P2 got them which means uncommon on the civilian side (for now).
You can just cut the back out of that defrost channel and go to TAP Plastics and have some lexan cut to the right size. It's just held on with double-sided foam tape. Had to replace the tape on part of mine. Another victim of the desert sun.
Deleting it will NOT be good. Defrost only exits the HVAC console in the very center. You will get zero defrost in front of the driver and passenger without the channel. Gunner will be real happy but everyone else will be blind.
You should also get some like 1/2" adhesive foam weatherstripping as you will need it for the bottom and sides to seal them against the dash and the cab side walls. It doesn't really look like you have the rubber edging that goes against the glass either..... could just be I can't see it in the picture but there is supposed to be a rubberized edging on the front to seal it against the windshield and keep the sheet metal from contacting the glass.
For that matter if you want it to work well you should dismount the whole HVAC console and replace the foam between it and the cab duct, and inside the box where the flapper doors seal unless you are swapping to an AC console in which case I wouldn't bother till that job comes up on the punch list.
These trucks enjoyed continuous improvements over their useful life and many of them aren't just for comfort. The A1 changes in particular improved the MTBF by like 300%. That's HUGE and the military recognized S&S for building THE MOST reliable thing they have ever purchased - by a landslide. Not the A0 though. That distinction goes to the A1+ trucks exclusively. A0 trucks were honestly a teething nightmare with a manufacturer that had no history of experience building anything that was road legal. Mostly they made mining and rail equipment.
The windshield I sourced from Omni has a black border that coincidentally is at almost exactly the same height as the top of the center dash… 
