Hmmmm - never heard of a steel shot trap load.

Considering the size at the bottm of the wad, I would either put a cushion wad down first, then the .570" to 575" on top of the wad, or a hard .530" ball at the bottom, with a hard .58 on top of it.

That 20 bore H&HR will drive that single .575" or .570" ball to 2,000fps fairly easily, but if you aren't experiences enough with loading, perhaps staying with what's in the books is a good thing. I'd go through Ed Huble's data to see what powders they used in the 20 and 28 bores. Too- you can go to Lyman's shotshell handbooks and have a peek at their data for balls and slugs.