A couple of things to consider.
Your musket is an original with an iron barrel and it's going to be less tolerant of high pressure loads, which is what you're going to get
with the "big" minie and hefty cahrges of hot powder. Round balls weigh alot less, but when tightly patched, they create about as much pressure as a minie with the same charge. As an example, in a .58 cal barrel, according to Lyman, a 530 gr mine and 60 gr. of 2F Goex creates 3,640 lbs of pressure. A patched .560 ball with the same charge creates 3,700 lbs. With 100 gr it's 6,040 for the minie and 5,900 for the patched ball. You'd think that with the lighter ball weight you could up the charge to get more zip, but pressures will go up. I tried patched .678 balls over 90 gr of 2F in my relined bore as a possible hunting load and they shot pretty well, but after discussing this on the N-SSA forum, a number of shooters advised caution, even with a relined bore. A steel barreled Armi is a different story.
What you shoot out of the musket may depend on what you want to accomplish. If you're looking for top accuracy, then you're going to have to go for a custom mould and all the accessories that go with it. And, be prepared to spend alot of time at the range. The sights on a '42 will drive you nuts, and the rifled bores are finicky.
If you just want to have some fun putting big hole in the paper, I'd go with the Rapine mould.
He can make it in a .690 diameter and you can experiment with paper patching to take up the windage. I shoot the service load of 70 gr of 2F
behind the Rapine minie and it does very well on paper and whitetail.
Duane