Standard load is about 60 grains 2f and a standard civil war Minie slug would be my choice. The rifling twist is a bit too fast for a roundball.
The rifling twist in your musket most likely is 1x72 inches, which is also a standard round ball twist for the caliber. Rifle muskets can be made to shoot round balls very well. The trick seems to be using relatively large ball diameters, say .570" to .575", and relatively thin patches of approximately .010" to .015". It is also usually a good idea not to use patch lubes that are too "slick. Appropriate powder charges for round ball loads for these guns can be found in practically any good book on muzzle loading. Many round ball matches have been won using one of these old war horses, so loaded.
If you truly want to shoot Minies, I recommend that you start with Lee's target "ash can" bullet. These bullets cast relatively easily for Minie bullets (which might not be saying much), they shoot accurately even with powder charges as heavy as 60 grains of Ffg, they punch nice clean holes in paper targets, and they strike like the hammer of Thor at normal black powder hunting ranges. Another bullet to consider is the Lee R.E.A.L. bullet. These bullets are much easier to cast than Minies and they can be made to shoot accurately, especially when a wad is placed between the bullet and the powder charge. This having been said, it is doubtful that any conical bullet will shoot much, if any, more accurately or kill any more effectively than a properly loaded round ball can.