The 520gr balls translate to about 1.2ounces so 1 1/4 load data would be a place to start looking.
I haven't done much smokeless shotshell loading in a very long time so others might be better able to comment on that. I do load magtech cases with black though and know that you can't get enough black into any cartridge to be dangerous in a gun that's safe to use. The question becomes: how much do you want this to hurt? You don't say what end use you have in mind.
Personally I'd choose the power level I wanted for powder, say 30 - 80gr of 2fg or whatever maximum the case will allow given the rest of the load column. Then a nitro card and a fiber cushion wad. Next I would experiment with different patching around the balls to get a tight fit in the case. To keep them from moving under recoil a little white glue around the top ball would glue the patch to the case or maybe an over card could be used but it might not sit well above the ball
If you are firing more than a couple rounds between cleaning some black powder type lube on the cushion wad or the patch would be wise. Or some could even be put it the space above and between the balls.
