Has anyone tried making the hole in their radiator housing larger to fit a larger radiator?

You can put the biggest core size radiator in an A body (early or late) and the size of the hole on the core support does not matter. The core support hole does not have to be the same size as the radiator core. As long as the majority of the core sees air flow, that's all that matters. You won't see an appreciable difference from an inch or two of the core blocked on each side.