Aerodynamic upgrades?

Wow, thanks for all the great ideas and discussion so far! For a little more background, the cam is an SRT Max Plus from Inertia Motorsports with a slightly different LSA than the catalog. I've only got one dyno chart for it and it wasn't great as I later found the car had a cracked pressure plate fork and the tune in general was very conservative (I did it myself and didn't want to push any numbers due to lack of experience). The car cruises 70 mph at around 1600-1700 rpm with the current transmission, rear gear, and tires, though I expect that number to go up a little bit when I put on my next set of tires as I currently have some ET street radials on the car that will be changed to a more highway friendly tire and I'll probably go to a common size on all 4 corners to make spares or moving stuff around easier. The EFI setup is a Megasquirt MS3X, so I've got full control of pretty much anything I want in the tune now, and I recently finally got a knock module installed in it, so I'm willing to push things a little further with the added information I can get from them. I'm planning to lean out my highway cruise a little more (currently running the typical 14.7, thinking I'll try to target more like 15:1 to start with) and will need to add in timing to compensate.

Lots of interesting ideas on the body to try to improve things. I know the recessed grille is basically the Achilles heel that I'll never really be able to fix, but some simple mods like closing out the core support to the K frame and maybe some Gurney flaps seem like easy enough things to try that could make a noticeable difference. I'm not really trying or expecting this car to ever be anything like a hyper-miler, more just seeing if I can get from an average mpg in the low 20s to the high 20s as I know that's what the modern cars with the Hemis can do. I know they are going to most likely have better aero, but not sure how much better. If I could get 5 mpg from a better tune and some limited aero mods, that would be pretty great for long term. I think a good 40+ miles of my 70 mile (round trip) daily commute are all highway, so that was the main reason I was looking at aero updates. Might have to see about some baffling behind the grille to better force air into the radiator opening instead of just randomly around the structure up there, but I know the gap is pretty limited, so not sure anything I could make would be a huge difference. Will have to just go stare at it one of these days and maybe play with some cardboard. I've got a 3D scanner, so might be tempted to see if I could get some CAD data going to play with as well, but that's probably overkill.

Regarding the Charger 500 rear window and Gurney flaps, would it be any benefit to put a flap at the back of the roof just before the rear window? Or does it only really make a difference at the very rear of the car where the air totally leaves the body? I think I could get some small aluminum angle bracket and attach it with some double sided trim tape easily enough as a test, but wasn't sure where to put it.