What do you need the finished dimensions to be?
That size board isn’t too bad to flatten on a SS jointer, as long as the finish width is under 4”. I just try to start the first pass at about mid-rock, so as take roughly equal amount of thickness off of the opposite corners.
If the board is over 4” wide, or if I have barely enough thickness to get it flat, I use a Thien planer sled. Quick and easy — all you need is a hot-glue gun, some wide masking tape, and a flat board. I usually use a piece of melamine shelving for the latter. You can tweak the workpiece rock angle while the hot glue sets up.http://www.jpthien.com/ps.htm