Totally agree about the Vibration. Before it cracked the block you could hear a slight buzz at certain rpm gone in an instant. I took expert advice afterwards the block cracked , they looked at the set-up suggested doubling the weight on the rear of the crank refitted a heavy flywheel that apparently moves the vibration. On the re-build the vibration was gone. We also suffered the belts turning inside out we always removed one for Topspeed runs to stop them taking each other off. Solution the same as you. We have not cracked another block since but Brunters is a stiff test. The MK girdle will help on the vibration front without taking half the strength out of the crank caps as it seems the crank caps are not the problem certainly my crank survived a 4 chunk block split & was measured to within 'as new' spec.
We can but try this solution if it works we will have a Topspeed record over 215mph if it dont a big bang but at least i will try.