Cold85,
First off, bad a$$ car but it puts you at a disadvantage to all the slippery late models. Soap box cars & bicycle racers benefit from aerodynamic aids so there's no such thing as "too slow to worry about it".
Look back at the way they kinda fixed the Charger 500's of 69.
Inset grill is an aero killer, do a full lexan cover @ the forward most piont of the grill to keep the stock look.
Rain gutters on old Mopars stick out like the doors are open, get rid of them.
The generic deck spoiler is too narrow, it needs to come all the way out to the edge of the fender & either roll down or have a fence on it. A row of vortex generators on the back edge of the roof will also help the hole behind the rear window.
Use some push on weather stripping on the back edges of the bumpers & get them sealed to the body or just finesse some gorilla tape in there to seal the gap.
As everybody has pointed out, it's a dirty bitch under the front. Get a chin spoiler on it & run a panel back from that to clean it up.
If you can get a little nose down attitude to it (while it's under power) that'll speed up the bottom air as well.
don't be too worried about getting it down on the ground where all the bellybutton Mustangs & Camaro's are, you're better off letting the old girl breath under there. The bottom edge of the rear bumper & roll pan are like scoops blend them into the body with some fiberglass or plastic sheeting. Lotsa race car stuff at Home Depot.