You're facing the classic dilemma of loading smokeless powder in a large-caliber, straight-wall, black-powder case.
A caseful of a faster powder puts entirely too much energy in there, and its burning raises pressures entirely too high, too fast.
A caseful of an appropriate slower powder can't burn to its best efficiency, because the large bore and bullet diameter lets the bullet move too soon, too fast for the powder to develop its full potential.
I do not know any good way to resolve this dilemma.
