The extra long cylinder space changes my recommendation to 340 grains, which will move out faster than a 320 from a standard Ruger Blackhawk at equal pressure, with more powder of coarse, but accuracy will be like a rifle, and you don't have to use all the powder space if you don't like too heavy recoil.
Bearing length of LBT bullets doesn't change with nose length, but only moves farther out of the case with increased nose length, which increases accuracy potential, decreases pressure with a given load, or allows more powder to be burned as the reloader chooses.