You will find a lot of useful information about your car at the following web sites:
The 1970 Hamtramck Registry Library Page (1970 - 1974) (technical documentation, dealership info, etc.)
Mopar Fuselage Styled Full Size Cars (high-resolution ads and sales brochure, courtesy of member
@ceebuddy )
Your interior is
M5L2 Beige, and the Hamtramck Registry Library would have the info about the correct color of the headliner for your car. FWIW, yours is one of the few 300 that I remember seeing with that interior -- see
here for some others.
@marko almost bout
CM43T0C266177, a 4dr hardtop (with the same FF4 green paint as yours) with that interior, and he may have photos of the headliner.
Five years ago,
@ImpJay (Finland) posted photos of a 2dr 300 that was for sale in Germany and had a beautiful M5L2 interior -- see
here.
@Dobalovr is a well known and very helpful FCBO member who owns CM23T0C197521 -- a 300 2dr (like your car, except his is ER6 Scorch Red) with the same M5L2 interior that he
bought in 2015 -- you may find his
restoration thread useful, including the pics that he posted of his car's headliner.
PS: regarding a headliner source, I have had good luck with SMS for two original material and properly cut, ready-to-install headliners -- but others have waited months and months to get what they ordered.