It's not unknown for makers to change specs of their cards in production but keep the same packaging, and some 7950s use 1x6 and 1x8 leads, my XFX 7950 for example does.
That powersupply is NOT going to be safe, even premium quality 400W parts could struggle with the demands of a HD7950, especially with a strong CPU so you really need to swap it for something of between 500 and 600 Watts with the correct PCI-E leads.
A lot of PSU units these days come with 6+2 leads rather than 8 pin, this is just for flexibility, the same lead can be used as a 6 pin or, by inserting the 2 pin part together the 6 pin part, as an 8 pin.
Look for: Seasonic, Silverstone, Antec, Corsair (not the 'VS' or 'CX' series), FSP (Aurium series), Rosewill (Capstone series), XFX (Pro series) or Fractal Design.