What is your purpose of boxing? Guys have been putting all kings of motors and mods in old trucks and cars before "boxing" was a thing. Never seen a broken frame except on a airborne ditch jump.
Truck frames were meant to flex. Boxing is for a hot rods without crossmembers and cut up frames with no suspension.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn