The main reason for boxing frames came from putting high performance engines in light cars such as model A F**ds. The frames on these early cars were a channel section and had rather limited resistance to twisting. A boxing plate was added to the channel to form a box section which is much stronger than a channel. The torque of a more powerful engine tended to twist the frame and lift the left front wheel. A permanent twist was left after a while and the frame was weakened to the point of cracking and eventually it would break. The pickup frames are already strong and no advantage would come from any added steel.