Jalo, based on a fair amount of patterning steel shot and using it for game, I would go with .010 (IC) and .015 (Lt Mod). This is about what the Old Colonel suggested and works very well for steel or lead. Steel shoots much tighter patterns than lead thru the same choke. DO NOT shoot steel larger than #6 thru the current tight chokes!! You risk bulging the barrel just ahead of the choke. I've seen a lot of these bulges in barrels with full chokes.
By the way, you have acquired a very high quality shotgun. The Miroku 500 is a true Anson Deeley boxlock, and is not the same shotgun as the Miroku built Browning BSS which has a thru bolt design. Enjoy.
Sid T.