Is it constant misfire on cyl # 6?
or does it do random?
Have you put a heat gun on the cyl brought the rpms up see what the heat reading is on that cyl compaired to others?
Have you done a compression check on all cyl ,leak down?
changed cap / rotor wires,plugs?
Just for grins , check the valve springs for breakage??
If it is an actual firng issue, change out the dist?