BB, preventing the cracking while still having the vertical vibration induced by the engine, hinges largely on increasing the stiffness of the rails. There are two main generic ways to do this. You could increase the number and/or depth of the vertical flanges beneath the deck, which is essentially what the single deck machines had. Alternatively, you could box in the existing rails, preferably by welding a second set of identical rails upside down underneath them and welding the flanges of the original and upside-down rails together. Adding a flat deck contributes very little, except in increasing the ability to withstand horizontal vibrations from the engine.
I haven't seen or heard of a twin-rail machine having its rails boxed in (with the captive nuts for the engine mountings on the inside of the boxes, of course) but it seems like the most effective solution, though of course it does change the appearance of the machine.
