i would just buy the bracket with the boxes already attached to it like the link i posted... you're gonna have to buy them both anyways...
the hardest part will be routing the wires from the outlets on the floor to the new box you install... more than likely you have insulation in your walls that (depending on how tightly packed) is very difficult to "fish" wires through... you'll need something like a fish tape or fiberglass rods to push down the wall from the new hole you cut to where the existing wires are. attach (tape) to the rod/fish tape and pull them up to the new hole.
this is after you cut the power off and un tap the wires from the outlets of course.... hopefully the builders didn't tack or secure the romex to the studs all the way down to the box or else you'll have a hell of a time pulling it...