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Boxing the entire frame of a '40/42 is a big job, and adds a lot of weight to the rig.
I'd suggest doing some reading here on MUD as to the areas of the frame that need beefing-up.
IMHO, inspect the frame very carefully. If you can sand blast it down to bare metal so much the better. Look for thin spots/rust holes in the frame rails.
It is common to box the front frame rail opposite of a Saginaw PS mount.
Check the frame near the rear spring mounts. The "fixed end" mount of the rear springs should be gusseted due to the stress of suspension flex.