Luckily my High Wall is the Carbine version with round barrel and a shotgun style butt stock....not the crescent.. which helps a lot.
Powder wise with the Black I'm running right up to the base of bullet maybe a slight compression which is what you are supposed to do.
You could maybe get a few more grains in there with more compression but don't think it would add much.
According to Alliant's specs. a 405 gr bullet with 49 gr of Black MZ gives 1364 FPS but don't know what barrel length they used.
So I have an actual weight 508 gr. bullet with 50 gr. of powder seated to same inside space as my other 405 gr. bullet.