I would just like to point out that when you bore a cylinder .020" over you are only reducing the wall thickness of the cylinder by .010".
Also, my understanding of the term "thin wall casting" is it refers to the modern casting technique developed in the 40's and 50's to manufacture cast iron blocks. It is not a direct reference to the cylinder wall thickness of a particular family of blocks. The thin wall casting technique was developed to improve the quality and accuracy of the castings while making them lighter at the same time.
The anecdotal evidence regarding Clevelands is that .040" is the max safe over bore. That doesn't mean a Cleveland can't be bored larger with good results. As Chuck pointed out it is all about the wall thickness on the thrust side of the (thinnest) cylinder.
This has been discussed at length over the years on the FE forum as well as the 460 forum. The advice with regard to a 460 is that .060" is the generally accepted safe max overbore but many blocks when sonic checked can go .080 while some can go as far as .120".