If you do box it have your boxing plate end with a 45 or 60 degree slope, not perpendicular to the frame flanges. This gives a transition area from the "can twist" C channel to the "no twist" boxed area. Boxing probably not necessary with a small block but it does give a nice flat surface to attach things like mounts, brackets, lines, etc. On a macho torque set up and big sticky tires I've seen cracks form where the boxing ends.
