There's your 35-40mpg aero package. With a 0.5 Cd and a 30mpg V6, you'll probably have to drop Cd by 40% to even hope for 35mpg on the highway. Start adding up the mods...
I think if you had the money and will, you could get a rear window made in the style of the first gen Barracuda that would come close to the aero template (ghosted). With generous radii and some care, it might even keep your flow attached. Even if it's just close, it will improve your aero a lot. But for sure, do the air dam, grille block and belly pan. If no window, try to find a spoiler that sticks up enough to touch that aero template.
Cool project!
Edit: you might even consider vacuum forming a custom headlight/grille cover in the vein of the 70's Celica racing parts... It would help aero quite a bit and look super cool.
Another pic