The most important thing with this won't be the "plugin" you use it will be the performance and language used in the voiceover performance. That will be the key to ensuring authenticity.
For instance, if you were to look at voiceovers or presenter performances in Australia, they would be markedly different today than they would have been back in the 1960's. Accents would be different, language and grammar would have been different.
Focus primarily on performance, then look at the technical side of the production. The technical side is easy in comparison.
Most voiceovers from the 1960's will have little production sound and lots of orchestral music going on in the background. Check youtube for loads of examples.