I would make a chamber impression by upsetting pure lead. I've never had much luck -- never had any luck, actually -- making cerrosafe castings in lever actions chambered for wheelgun cartridges. The cerrosafe tends to get stuck, then you have to melt it out by dunking the action in boiling water.
The pure lead impression will tell you where the rifling starts and should also give you some information about diameters.
If you have any sort of 480 bullets on hand, you should be able to make a dummy cartridge or two and get a feel for what the gun prefers for COL and nose shape.
There's more to it, but that'll get you started.