Your souvenir sheet is SG MS5092, 26th Soviet Communist Party Congress (2nd issue). I don't know what the quote says, but I see Brezhnev's name at the bottom of it.
The last few paragraphs of this article give a good idea of what you have on the overprints.
http://www.linns.com/howto/refreshe...rcourse.aspxQuote:
Lastly there are the overprinted Soviet stamps lacking any postal validity that have been produced primarily for sale to stamp collectors.
Ryan