You guys are way over thinking this question. First & foremost, the primary issue is no different that non-can hiking: pack weight distribution. That means, as Bob mentions above, your food should be somewhere in the bottom 1/3 to 1/2 point of your pack.
Once that is settled, the next question is how long will you be out w/o re-supply - this determines the size of can you will need. Once you have the can size determined, you finally arrive @ your pack. If your current pack cannot handle/hold the can you need in the bottom 1/3 to 1/2 point, either (a) get another pack, or (b) revise your re-supply logistics ie move to a smaller can.
I know a lot of people do not want to acquire yet another pack, but the money will be more than well spent. There is simply no comparison between spending endless days constantly d!cking around with a poor set-up (shifting, slipping, top/bottom heavy, etc), perhaps to the point of even being uncomfortable, versus smoothly cruising with everything under control.
One last thing, if you decide to use a BV450 (it's what I have), do as Bob does as well: place it sideways with either the top or bottom facing you. It turns out the BV450 is actually slightly shorter than wide, so this minimizes the extension. Another benefit is that it places a flat surface against your back/pad, vs a narrow band of the circumference if the can is upright or sideways (top to the side).