Welcome to History Hub,
The gravesite and obituary of your grandfather at findagrave.com
Contact the following to get information about death certificates
Newellton Town Clerk
1874 Louisiana 605
Newellton, Louisiana, 71357
Unless he died on a federal installation, or on a federal ship at sea, the National Archives wouldn't maintain those records.
You need to contact the Louisiana State Archives. You're in luck, under Louisiana state law, death records are confidential for fifty years after death--meaning, in this case, until 31 December 2021.
You can order a copy of his death certificate here for $5, or $10 if you need a certified copy:
Dear Ms. Cullen,
Thank you for posting your request on History Hub!
Death Certificates for the state of Louisiana that are less than 50 years old are in the custody of the Louisiana Bureau of Vital Records and Statistics. For information on obtaining them, please visit their website at State Registrar & Vital Records. For records that are more than 50 years, Please contact the Louisiana State Archives. Since your grandfather died in December of 1971, we suggest that you contact the state archives first to request a search for his death certificate. You may wish to note any name variants or broaden your search for the surrounding years. In addition, the FamilySearch Research wiki for Louisiana may be helpful.
We hope this is helpful. Best of luck with your family research!