500 SS are very difficult to get in oversizes. There are some SS 600 oversizes in the Holder stock now held by Graham Moag (Crown stamped T)
I had this same job to do for a customer recently and had to do a lot of modification to an odd pair I found.
Contact Ken Lack, as he has a few piston patterns and may be able to make a pair of cast pistons for you. Be careful before boring to 600 as some barrels had smaller cores to suit the 500.
If you do overbore, it might be prudent to use 600 LS standard pistons, which obviously smaller than 600 SS (Graham has plenty) then weld up the crown as it should be 1/16″ higher for a SS, remove the ring stops and machine the OD down as far as leaves a safe wall then deepen ring grooves (with care) and refit stops. You will now have an engine of about 560cc. As you can tell, this is a somewhat lengthy proceedure but at least you get a die cast interior rather than a cast one that tends to be heavy and the conversion is less work than making the whole thing from scratch and a better result. Best of luck if you do it. It needs care and time, but like anything, it can be done! Roger