I have seen a number of 70 R/T's with either the body color or black painted rear tail panel that surrounds the taillights. Which is correct or is there some kind of date cutoff for this?

Body color on R/Ts except for the cars that got the argent trim panel. Black was on T/As. There are probably exceptions to the rule though.

Argent on a 70? On a 71 I have seen.

Yes, a '70 R/T SE has an argent tailpanel.

Ah yes you are right
A63 molding group.

My old A66 car, original paint had the A63 Molding Group.
As stated there were exceptions, plenty of survivor RT examples that are black.

For all 1970 Challengers;

Blackout on T/As only.

TX9 tail panel paint on TX9 painted cars - except T/As. (see above)

Argent on the aluminum tail panel trim for SE models and other models with the A63 molding group.

Correct or not, If the car was red, I'd have a black tail panel. Easier for people following to see brake and tail lights.

NICE! What engine?

A66 70 Challengers are 340 cars

I would have to look but think it is the medium gray not the light are dark argent.
I have 2 NOS panels here. Not the Metro repops.
Does anyone make a paint in the right shade?
I shot my grille in black organsol but may shoot it to match the tail panel.

Plus, on an R/T, I think it looks cool to match the black out on the grill.