A .530 RB to use in a .54 cal rifle weighs 225 gns out of a Lee mould. I use 55 gns powder for paper target comp shooting and for fun with the gongs at Lady Noisy's bush farm.
One of the neat things about a muzzle loader is that without the constriction of a cartridge case you load 'em up or load 'em down as you please! Most of us though, have just a couple of favourite loads and stick to them so we know where they shoot. My 54 shoots 50mm high at 50 yards so I can use the traditional "6 o'clock hold".
Lorgar the 420 gn bullets you're thinking of (and had a play with) are used in my Sharps. Launching such a big bullet does generate quite satisfactory recoil and with most of these big bore rifles you don't want to be shooting from a prone position - even using a bench at the wrong height will start to hurt after a short while!
I had to mess around with height of my milking stool (that I use when we shoot off cross sticks) so my collar bone wasn't getting hammered....kind of "good" hammered though !

There's still a few of Wile. E Coyote's ideas that I haven't tried yet.