If stress points are present due to cracks, boxing isn't likely to make them go away. The frame was designed to flex to some extent, and making it totally rigid will increase the possibility of a break, IMHO. I can see the advantage of doing some limited reinforcement at stressed areas like add-on motor mounts or crossmembers where they weren't originally engineered to be, but I doubt that doing a full boxing job is necessary. If you're installing something with a massive amount of torque like a big block V-8 or a high torque Diesel engine, why not just fabricate an entire frame out of rectangular tubing, using the original frame as a template?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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