I have a .54 cal mold sitting here for my .54 Hawken, but I've never fired any of the bullets, because I can't get a good bullet to drop from that crappy mold.
Anyway...
While trying to decide between a few options for a conical bullet, I did a lot of reading on the REALs (.50 and .54).
Reviews are very mixed.
Some people love them.
Some people can never get them to work.
Twist rate matters.
And most people that
do get them to work swear by a lubricated wad under the bullet.
With your 1:48" twist rate, I would expect the 250 gr .50 cal to be the best option. The 320 seems to only do well in faster twist barrels.