Boxing will not, in itself, cause stress risers.
It sure will if you don't box the entire frame member end to end. If you box a short section (close the open side of the C channel), there will be a stress riser where the closing plate terminates, especially if it terminates in a right angle to the frame. Jerry's comment about putting an eliptical end on the boxing plate reduces the stress where the plate ends by incorporating a little bit of flexibility there. And a 4-link mount on the boxing plate is going to introduce some twisting to the C channel, boxed or not. It WILL eventually crack.
