The factory used to recommend rings be no more than .010" larger than the bore and be filed. The only one I did was a factory .020" over bore 318. I used .030 over rings and filed them to fit. This was in the 1980's and they only place that I could find .020 over rings was order them from the dealer at about $95, I got .030 over rings from NAPA for about $30. To my knowledge, the engine is still running today. We routinely use rings that are .005" over (and file fit) on race engines.
Brian