Saskguy- perhaps whomever is sending this gun to you could e-mail you what the bore dimensions are. For shooting cast, you really need to know whether it is a .406, .410, .412 or .416" on the groove. The longer bullets emulating the 400gr. factory solid have long bore-riding sections. These need to be bore size.
Perfect regualtion and full power loads are possible - but they have to fit well or they won't shoot well.
Straight WW bullets are capable of handling up to about 2000fps with appropriate slow powders keeping pressures low. If hardened and possibly tempered to the correct hardness, they'll take velocities well over this - actually right up to 3,000fps and beyond. Hardened WW's are not brittle at 2,100fps and will give identical performance to FMJ's or can be made with soft points for expansion. The world of cast bullets has come a long way, thanks to people like Veral Smith of Lead bullets Technology.
You need the correct bullet before you can load, but you can have your brass ready - if you know the size of the chamber. It is not fun to have a bunch of ammo built, only to find they don't fit when the gun arrives - been there.