Hello, Ken:
Congrat's on the original Spencer.
As a SSS Member, you might Search here and read the "SORI" section.
Short answers:
Call Dave Gullo at Buffalo Arms.
He should have a proven mould.
Accurate Bullet Molds in Salt Lake City sells a popular bullet mold #51-320C.
Have it bored to cast in Lyman NEW #2 alloy, which is approx.16 Bhn, to an AS-CAST diameter of YOUR Spencer's groove diameter plus .002", and then size it to .001" over YOUR bore's GROOVE DIAMETER.
SPG bullet lube works well with black powder.
Please AVOID ANY smokeless loads in an original Spencer.
Cases can be from Rocky Mountain Cartridge in Cody, WY, but they MUST be reannealed after two firings to avoid mouth brittleness. Alternatively, and much cheaper: Get STARLINE 50-70/90 brass, and cut them to length.
They, too, will need annealing after a few shots.
The Starline 56-50 cases are for the Armisport reproduction and have a slightly smaller RIM OD, which can cause extraction issues.
Ballistic Recreations in Canada can supply their SALT BATH annealing gear, which avoids using open flame and damaging cases.
Dies can be CH4D, which Buffalo can supply. The rest is in SORI.
PLEASE keep us posted on your progress. Thanks.
El Supremo/Kevin Tinny