I'm not sure about the efficiency of your octabox but I can tell you this...
I use a Nikon SB-800 Speedlight which I believe has a slightly lower guide number than a Canon 580EX(II) and I have no trouble using a Photoflex 36" x 48" softbox, achieving apertures of ~f/8 at a distance of 6 feet from the subject and ISO 100.
If you need to shoot at greater distances or want to work at smaller apertures without raising the ISO then you're going to need more firepower but if your camera performs well at ISO 400 and/or you are working on tighter shots then you'll be fine.
Once you want a large light source at 6 feet or more, for full length or group shots, then you're really going to want to use more than a Speedlight or small flash. A studio strobe will make much more sense, and as 2112 indicated, he is using a B800 (320ws) at half power to get the numbers desired.
As far as I know, unless you want to rig something or or do a DIY, there is no practical way to attach or mount a small flash to a PCB octa or square softbox. If you're not using the foldable boxes you can opt to use a third party speedring or bracket assembly to mount a small flash to a box, but that also has its limits and will not work well with a large box.