Mike: IMO, the conical should be about .001-.003 larger than the chamber.
Dixies catalog says the chamber on the Mellennium 1858 is .450 diameter and they recommend using a .454 diameter roundball.
If my guess about the size for conicals up above is right, that would make the diameter .451-.453.
Although you mention that there is a .450 diameter bullet available, that would only be a line to line fit without any sheering of lead. In fact, depending on your exact chamber diameter it could be a loose fit.
A loose fit not only may cause a chain fire, but the unfired bullets will move forward during recoil and probably prevent the cylinder from turning.
If the conical is too large (or too hard), it will be a real problem to load it with the guns loading lever.
As a passing observation, the bullets that are cast using one of those brass bullet molds that comes in a cased pistol is a heeled bullet, that is, the base of the bullet is undersize so it will pilot itself into the cylinder during loading.
If your looking at bullet molds, you might see if it casts a heeled bullet.
I've cast some bullets using a .36 cal brass mold like I mentioned and even using pure lead, they take a LOT of force to get them sheered and seated into a cylinder. When seating a round ball, the cylinder sheers just a little lead ring off of the ball, but when seating a conical, you will have to sheer the lead for the full length of the slugs cylindrical area.
zonie
