Wow, with that much torque, and that much tire, there must have been a ton of twist on those rear rails to cause a stress crack. It almost looks like the welding in of the 4-link may have weakened the frame rail and made it brittle along the weld edge.
If it were me, I would think the best way to repair would be to get the cars weight supported so that the cracks close up and then re-weld the crack. I would then probably section out the outside of the frame rail and add steel to the inside of the channel to re-inforce the rail. After that, close up the outside of the rail and maybe add another plate of steel to match the rail profile and give it some more thickness.
Without removing the 4-link, the access is pretty limited. Grafting in heavier rear frame rails might be in order during the off season.
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69 SS396 Camaro Coupe, 4link, 12bolt posi, Speedtech LCA and UCA's, Hotchkis 2" drop springs and sway bar, Ridetech triple remote resevoir coilovers, Bilsteins, Wilwood 6 piston front/4 piston rear, Magnaflow stainless exhaust, Lemons Headers, Comp rollers/p-rods/cam, Fikse 18x8.5 and 18x10
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