That's a good looking English pattern rifle, for sure and in nice shape.
My current big game rifle is a 14 bore - that's .69, on a 1 1/8" straight barrel 31" long. It weighs 9 1/2 pounds which is perfect for a large bore. The 14 bore is as large as I'd do due to velocity/recoil matters. Any larger and recoil will keep your loads to where trajectory becomes poor. A 16 bore would be absolutely perfect- for a 1/2 round, 1/2 octagonal tapering from 1 1/8". I'd bet Ed Rayle would make that barrel. He uses .675" bore which is actually 15 bore, so you can use the 1ounce ball - .662" standard Lyman mould.
Dan has just such a rifle.
In a .41" round ball rifle, I'd use a .400" or .405" ball if I could find such a mould, with a .020" denim patch.
In a .410" fast twist rifle barrel, there are a number of .41 cal. pistol bullet moulds that might work perfectly, as cast in pure lead. Searching pistol sites to find a .410 mould that casts .001" oversize (over bore size) in pure lead would be a good idea, or simply enlarging the mould yourself if needed. If the twist is slower than 26" and faster than 40", you'll want a very short bullet, in the 160 to 180gr. range, I'd guess. That's for shooting a naked, grooved lubricated bullet.
If wanting to shoot a picket bullet, ier: cloth patched, Dan and Steve are much better equipped at suggesting these things.