Easiest way to cut, shave, or chisel bottom 8" of posts to fit footings
You're going to shave the sides, not the bottom.  The stability will come from the down force on the bottom of the post - a milimeter shift laterally inside a mounted bracket is nothing.

That said - easiest approach IMO will be to set your circular saw to an extremely shallow depth and notch 8" height to that depth on TWO sides, not all four - and repeat that notch from the top line all the way down to the bottom - cutting over and over and over.  Then use a chisel to knock out the rest of the wood and clean it up some.

Something like this but with a REALLY shallow depth on your circ saw.

http://www.youtube.com/watch?v=B0GJw7y9vzc