I was planning on using the Hornady Great Plains bullet this November, I believe its a 385 grain conical...
That rifling twist will not stabilize conicals. You will be using round balls or perhaps ballets.
In my 50 flinter(1:66", 39.5" OAL), I used 70 grains of Swiss 3F, a .495 ball and .015 moose milked patch.
In my 54 percussion (1:48", 28" OAL), I use 110 grains of Goex 3F, a 535 round ball and .015 moose milked patch.