A .62 calibre needs no slugs. The patched round ball is sufficient for all game you list, including Alaskan & BC moose. To increase the power of a muzzleloading rifle, increase the bore size, not the weight of the slug. A larger bore not only produces mush more dramatic results on game, but recoils less than the same weight slug from a smaller bore.
: A .715" ball weighs 545gr. and is sufficient for Tiger, Water Buffalo and Indian Elephant, not to mention most African game.
: There is a fellow on the www.americanlongrifles.com who has the underhammer rifles you speak of, right up to .73 calibre. You could go there and ask him.