With the 18" twist, I would look to a decently long bodied, grooved, like a Lovern design, short nose, 5 to 7 grooves.
For powder, likely 70 to 90gr., whatever the gun demanded. At some point, the obturation of the bullet will be perfect
and the bullet will be accurate and shoot cleanly, without leading due to gas-cutting blow-by.
No, all the long scopes made for the long range shooters, are barrel mounted, seems to me. You may have to make up
a mount that duplicates the one on your rifle.
NOE Bullet Moulds has some lighter weight .454" bullets, but most of those designs are not for black powder. Black Powder
grease grooves are usually larger and square in shape, as-with the Lyman #457125 mould, with 4 square grease grooves.
That is the round nose in this picture. The pointed bullet is the Schmitzer bullet, designed for silhouette shooting. It has 3
square grooves.
I've never cast these in a soft alloy, so I don't know the size they would cast with suitable alloy.
You can click on the picture a couple times to enlarge it.
Now, Lyman did make a .451" Lovern Design, short nose lots of square grooves for the Parker Hale Volunteer rifle, that might be a good one.
It would have to be bumped, maybe, but I don't know what size it actually cast. It was labeled as Lyman #451112 (275gr. to 485gr.) or #451114
(450gr.), I think. Whether they are still made today, I don't know. I would check NOE, Buffalo Arms, NEI and anyone else you can think of, even Lee,
except theirs have smokeless powder lube grooves. I think a long bearing surface bullet will be best, as nose riding designs are critical of nose diameter
in how well they shoot.