Larry,
Drill holes at either end of the crack to prevent the cracks from spreading further. A welder can fix the cracks. Just fix what's visible for now, and see what happens.
Now ask yourself why that header is cracking in the first place, and correct the cause, or it will continue to crack.
About the only hint I can give you, exhaust systems do not like to be rigidly mounted, that's why they are always hung with rubber or cables, never fastened to the chassis with solid mounts.
Another possibility would be b/o motor mounts allowing the motor to flex in the chassis.
Good luck buddy.
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