Tried 350gr cast .458 (45/70 bullets) in MMP orange sabots in my 1-48 NewEnglander this fall and was pleasantly suprised. At 50 yards the first 4 shots produced 1 ragged hole and the 5th shot went about 1/2" high/left. Still a very good group for open sights for me!
About all I can tell you is try a few combinations and see how it does.
BTW - the 350gr .458 is a fairly long bullet with a RF (round/flat) nose.