Cracks can always be repaired in most anything.
And the how its done depends on what its to be used for,
and where the crack is?
If it was mine, and not to be used for heavy hauling and hard use. I'd just drill the end of the crack to reduce the stress, then use a die grinder and grind a weld prep and weld it all up and grind it nicely and paint it.
If it was going to be used of roading and heavy use. I'd do the same as above and then insert a frame section that would fit inside the frame extending a foot plus both sides of the cracked area, and drill through the sides not the top and bottom flanges, and use 1/2 grade 8 plus bolts maybe 6 per side of the crack staggered. The insert has to fit proper in the existing frame, and maybe difficult to find a section that will properly fit. Anyway that is the correct way to fix it. You could also plug weld the insert in. Fish plating is another approach paint the inside areas first with either way.
A full weld on the insert deal may not be a good thing as any vertical welds are a good place for more cracks to start. Better to seam seal it.

Kicking self for selling off my Taskforce trucks.
Still looking for an LCF or conventional big bolt in decent shape.


As of 10-26-2022, A 55.2 Taskforce long bed now the work begins