Magnum blocks

There's a lot mention about the magnum engines being good for a budget build. So I started to do research on them and one issue that arises often is that the magnum blocks should be limited to a maximum over bore of 0.020" because of the block's being a thin wall casting, is this fact or fiction? I thought I read somewhere that the block's have a thin cylinder wall in order to save weight, that little bit of metal isn't going to save that much weight. If I was asked to design a engine I would have it to run as cool with a 0.060" over bore as with a standard bore size.