The overture was present when I saw it back in '79. I missed it on all the 80’s video releases.
Not sure if I still have it, but the Anchor Bay VHS box set came with reproductions of the lobby cards and a printed interview with the director, in which the alternate ending is discussed.
http://www.ebay.com/itm/The-Black-Hole-VHS-Movie-Limited-Edition-/141802565196
I definitely still have the early 80’s Laserdisc if you need the original Dolby stereo mix.