It is worth studying what GM found and patented about covering only about half of the open bed, strangely from tailgate forward:

https://patents.google.com/patent/US4573730A/en?oq=4573730

A professional aero guy would suggest that you should tape 5 inch long strings of yarn on the outside of the rear window, inside the bed, and across and down the tailgate. Then find someone else to coast your truck down the long hill while you video it. The video should show signs that you have a horizontal “whirlwind” behind the rear window, and a second whirlwind behind the tailgate. These whirlwinds “break loose” every so many seconds, followed by a new whirlwind forming. You want to make mods that “ lock the two whirlwinds into permanent attachment.” When the whirlwinds stay in place they create an “invisible sloping camper top” .

The V12 SRT Rams had blacked out front grille blockoff plates that removed at least half of the opening.

For your own education, coast down your hill with the mirrors folded flush against the side windows.

If you were a weird risk taking coal miner
you would coast down your hill with a tire air pressure 10% higher than max rating, but only on a dry day with nearly no traffic.

I did not know this until recently, but when I read Wheeler’s US Patent I found that he states that labeling tape with capital “V’s” impressed side by side function as tiny vortex generators. This might be an interesting experiment on mirror back edges, and perhaps where the glass windshield” meets the metal roof edge.

Why not try a deeper front air dam that is flexible, and perhaps a second, third or fourth dam placed underneath the truck, instead of an underpan.

Examine the “front spads” that some SUV’s have that block only half of the rear tire width.

I am now 90% convinced that a lot of the odd shaped rear brake light lens designs
are that shape to hold a whirlwind stable and attached.

With your junk yard connections could you get a add on overdrive unit to experiment with?

If you succeed at aero power reduction,
you need engine rpm reduction to get full benefit.