The GM engineers must have figured a rigid box frame can be connected to a flexible "C" channel frame without giving problems. Note on this factory S10 frame the splice (half way between tall cab mount brackets) has a long parabola (Nike "Swoosh") shape cut out to give a rigid to flexible transition area. I'm sure a boxing plate with the right shape is also trouble free. It's the square ends on plate that set up stress cracks. Also, DO NOT have the box plates of thicker material than the frame. If bolts need to be torqued to where they will cave in the boxing plate then weld through tubes from outer to inner surface to take the torque without collapsing.
[img]https://i.postimg.cc/JHRtmTyy/S10-028.jpg[/img]
