You are hearing all the right tips here.

Your ball & shot came from a very reputable maker, therefore, it was very, very likely well regulated and accurate when new. IF the bores, sights, stock, etc. are more or less original, then it is safe to assume that is still regulated and accurate when fed the right load. The trick is to figure out what load works. There was an article in DGJ by Ross Seyfried where he used one of these to take a cow elk. I do not recall if he discussed the load and sizing relative to bore dimensions. I do know that the bores are tapered as RM mentioned and it works with a Fosbery type projectile as also pointed out.

I think I would first slug the bore just ahead of the chamber, then do it again, but push the slug all the way through. This way you have dimensions just ahead of the forcing cone and at the muzzle. I would do the muzzle, just to make sure there is taper and your bore is not altered. A slug from an oval bore will be squashed and oval in cross-section(excuse me if you know this) with minimum dimension one way and a maximum dimension 90 degrees from the minimum. I would take the maximum dimension, just ahead of the forcing cone, and size my Fosbery projectiles to this dimension. You will have to cast, or have someone cast, the Fosbery slugs out of 15:1 lead:tin alloy, then coat them with liquid alox. Nei offers a paradox mould that seems to work for a lot of people. It casts .740 dia paradox slugs that weigh 740-750 grains.

The load that works will probably be in a 2 1/2" case (cut down federal paper or cheddite plastic), have nitro cards, felt & cork wads and either a roll or "fixed" crimp. The powder will most likely be Unique, but possibly black or Blue Dot. Velocity at the muzzle for the regulating load will be between 1050 and 1150 FPS.

Next question; do you have loading experience? If so, we can go deeper here. Also, there is a ton of older load discussions on NE and in DGJ that would be a huge help to you. The last guy that had a Lancaster B&S just called Ross and I guess he got what he wanted. Good project.