score:6
You probably mean “the letter of 13” (not of 30), which indeed has the date March 25, 1966.
The Russian Wikipedia has the articles on both letters, including the signers lists and other details. They also refer to the internet publications of the full transcripts
All publications refer to different pages of "АП РФ. Ф. 3. Оп. 24. Д. 495", that is a particular dossier in the Archive of the President of RF. I guess thus that the primary source is not easily accessible.
I have not seen any full English translations online, but the letters are written in the standard official style, so the automatic translation mostly produces reasonable results.