If you do this, save 2 inches or so of the same barrel you made for the gun to make yourself a push through sizing die. Run the bullets through a lubrisizer to fill the lube grooves, then through your barrel section die to size and pre rifle them. If you don't, you won't be able to load them from the muzzle. You could get a mould made up to cast bore size bullets that can be loaded, but lightweight ones like you want don't bump up very well unless they're hollow based.
You also don't need a breech plug. Just use a 357 case. I normally shoot my Sharps 45-70 as a muzzleloader, just prime a case and chamber it, then load from the front. The trick is to use enough powder or a combination of powder then filler to make sure the bullet is in the bore, not the throat. I get considerably better accuracy from my rifle muzzleloaded than I do with fixed ammunition. Also remember this won't work with smokeless powder. The bullets won't bump up to seal the bore and it will lead horribly. A handi rifle would work fine for this, but I wouldn't do it with anything that didn't have an exposed hammer. No need to shoot yourself, y'know.
