[stanz]

Not sure that's gonna help much, I would guess you create a vast amount of turbulence between the cab and the box. I have the EXACT same setup on my F150.

If you want to really improve the flow you need to create a transition fairing from the cab to the rear like on the big rigs. It should attach to the cab and taper to the rear. If you don't want to drill into the cab, mount it to your boxes and use foam to filled the gap to the cab.

Not rigid foam, flexible foam, like you use to install window A/C units. They used to sell foam pieces to go between caps and cabs back in the day. I dunno if they still sell them. The cab and bed of your pickup flex at different rates so the link can't be rigid.

I think the diamond plate finish may actually improve the flow of air over the boxes. I recall the "sharkskin" swim suits from a few years ago that weren't smooth. The ridges created a laminar flow with less drag than a smooth suit.