Longknife, if you make mods to the bullet I also would opt for a slug with a flatter nose like Datyl S says. In fact, I would agree with his entire post.
A 1-38 or a 1-35 will be fine. A bit tighter can't hurt but there is no reason to go super fast. Most of the slugs we are talking about there are effectively "large revolver bullets", but not fired from a revolver. However the S.D. of an LBY style revolver bullets is very accurate in magnum revolvers and if fired from rifles at the same speeds the same math comes into play. A twist of 1-18 is used in a 357 mag many times. And the larger the diameter of the projectile, the less twist it takes because as I said above, it's the function of diameter X Pi that describes the helix of the equator of any bullet through the air. So if you were to ass 40 -60 grains more I believe a twist of 1-38 will still do the job. If in doubt tighten it up to about 1-32 to 1-34. But be sure the strength of the lands of the bullet are sufficient. As stated above, the grooved of the bore becomes the lands of the bullet, so narrower barrel lands and wide barrel groves are going to give excellent purchase on the bullet.

Keep the shank of the bullet about .001" under land size so you can load it with a rod. If I were making the mold I'd recommend making the top land of the bullet (last to enter the muzzle) only about .002" over groove size so it cuts on entry, but a tap with a small mallet will engrave it and then it seats easily. Have it made only .020 Tall.
Cast from WW metal and air cooled a bullet will obdurate fine, even if harder then pure lead. Bullets, when made too soft, actually do not kill AS WELL as balls from muzzleloaders simply because the terminal path through the bodies of animals often veers off when the nose smears badly. But hard balls go straight.

As I said above, my 62 cal flintlock exits all the game I have ever shot with one exception. A buffalo, -----and that one was still almost an exit (under the skin on the off-side) and was still in a straight line. A bullet with an LBT style nose, case semi-hard, will do anything you'd ever ask of it.

For an excellent overview and probably the best info you'll ever find on the subject, read "The Sporting Rifle and it's Projectile by James Forsyth, written in the 1860s. He found out 150 years ago what I re-learned for myself 40 years ago. Balls actually do a better job for most hunting. The gun he recommended the most was actually the same type as I have shown in the pictures above. It fires a ball of 1 Oz (437 Grains) and from the muzzle to about 100 yards it shoots where it points. past 100 yards you need to hold the sight picture a bit higher, but out to about 150 it's a very easy rifle to make kills with. Once you go out past 150 is where bullets start to overtake balls, but the in the real world about 98% of all you kills with a muzzleloader are at 125 and less.