The problem you are having is called bumpsteer. I had a stock 1931 model a that had the single leaf and split wishbones. I would have that same problem of the steering wheel shimmering if I hit uneven road surface. The problem you are having has been discussed many times in hotrod articles over the years. I think the majority of the problem comes from the binding of the steering system via the steering gearbox and what type steering arm you are using. For the best answer go to a search engine like google and type in "bumpsteer". After you have read several articles you should be able to remedy the problem.