Some thoughts I'll share, everything vibrates, me, you, your house. We aLLhave a "Natural Frequency" that can be exiced to the point it causes damage. I once got close to a large industral blower that had a damaged silencer on the intake side, the closer I got, the worse I began to feel, back away and the bad feeling would go away. The frequency of the sound it was eminting was messing with the natural fequency of my body.
I work in a large industral enviroment and we have problems with big machinery vibrating, mainly when we try to increase production by running the machines faster than they were designed for. Speeding them up almost certainly guarentees us of a problem. One thing we do is to add mass, increased mass asorbs some of the energy created by the vibration. We do this by filling the support beams with concrete. We also add rigidity, we do this by increasing the amount of metal in the structure, this raises the vibration point and allows the machine to run nice and smooth.
When Briggs built the 28 ci OHV, it was designed to run well below what we do with them. it was also designed for 28ci not what we have them out to now. Personally I beleive (along with the Vibe Techs at work) that case flex is a real factor in these cranks breaking. We have a program that can be atached to a machine and it will read the amount of deflection occuring in a machine. This is transfered to a moving picture and the results are amazing! I'd be willing to bet bothe the top and bottom of the cases that support the crank are moving all over the place.
How do we fix it? Add mass or rigity. The support bearings on either end will help, but we feel not cure the problem. There is very little room inside the block to add "Gussets" (triangle peices of aluminum) Rules are specific that the outside must "apear stock". So I don't really know, a billett block like "Blockzilla" might do it?
Just rambling thoughts.