We box a lot of frames but NOT for stiffness. When building hot rods the smooth boxing plate from the cab to the front crossmember is much easier to work with than a "C" channel when it comes to custom mounts, IFS, steering components, etc. The boxing plate MUST NOT have square ends because this is a sure way to flex stress cracking. You can use a parabola cut out on the ends of the boxing plates or a 60 degree taper angle. This gives a transition area from rigid to flexible. Peek under an Chevy S10 or GMC S15 and you will see this has been done where the front box frame connects to the rear channel rails. Believe it or not GM actually does have some pretty smart folks solving engineering problems. A 70 GMC and a 52 Chevy both with @ 450hp Corvette/Camaro LS engines, "C" channel frames, and no boxing plates. GMC has a lot of rubber on the rear.

[img]https://s22.postimg.cc/xp07m6kvh/IMG_4118.jpg[/img]
[img]https://s22.postimg.cc/ejwyckbe5/IMG_4398.jpg[/img]

Evan