The Church's plan for general conference archives is described in
this article. It explains that video and audio (and text, of course) are available back to April 1971.
At the beginning of the April 1970 conference, Boyd K. Packer was an Assistant to the Council of the Twelve. He spoke on Saturday, April 4, 1970, at the Priesthood Session. The text of that talk is available on page 40 of the
April 1970 Conference Report. He was sustained as an apostle on Monday, April 6, 1970, and he did not speak again during that conference, so his first talk as an apostle was not in April 1970.
His first general conference talk as an apostle was given on Sunday, October 4, 1970. It can be found on page 118 of the
October 1970 Conference Report. I don't know where video might be available.
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