I had a good look around at some other photos of cracking in SB's and did some thinking. The cracking is caused by lateral forces on the engine bolts being transferred to the captive nuts, as intended in the original design. The twin rail design is a significant improvement over the solid deck as they are allowed to twist and move independently of each other which lessens some of the cyclic vibrations into the complete frame. Stiffening the rails would remove this flexible feature.
The problem lies in the transfer of the lateral force to the rails. The connection between the captive nuts and rails is simply too small and incorrectly designed. All the cracks I have seen propagate from the two welds on each captive nut.
The best solution would be to remove the captive nuts, repair cracks, replace the nuts with a 5-6mm thick plate sized to fit between flanges of rail and 50mm long, drilled and tapped for engine bolt fixing. The 6mm plate needs to be welded in place of the captive nuts on all 4 sides with ~10mm long fillets in the centre of each side(not corners). That should allow the rails to do what they were intended to do.
