Never had a long barreled .54, but I did have one's with a 36" barrel. I used 85 grains of Ffg, .015 patch and a .530 ball. That was a prize winning combination for me, back in the day. Now? Don't ask.
As far as hunting for a bull moose goes, if it were me, I would trade that .54 in for something in the .60 to .70 caliber range. You are going to be hunting something that weighs in between 1,000 and 1,400 pounds, more or less. And you are going to be going after it with a 230 grain round ball that is barely over 1/2" in diameter? I know, it has been done before and it will be done again, but it can surely be done better. I think back on the bison I took with my .72 caliber sporting rifle. 100 yards or so, 200 grains Ffg, 555 grain round ball, dropped him from a standing position to flat on his back with his legs in the air. Pretty amazing for a 1,700 pound critter.
So, bottom line here is that regardless of the rifle caliber, you want to develop the most accurate load for the rifle that you can, and you will want to shoot at a range you feel comfortable with. Hail Mary shots really shouldn't be taken.