i FINALLY got a hold of my brother-of-the-heart, C. E. (ED) Harris (Ed has been in Europe most of June & July, shooting in rifle/pistol matches, playing with the lovely ladies & stuffing himself on GOOD FOOD.) & he said to tell you that my head was full of mush, unless you like CLEANING rifles more than shooting them.
Ed said, "Tell your friend that any 375-410grain soft lead slug with 13-14 grains of UNIQUE is FINE for your carbine, without "filler" or anything else & it would be CHEAP to shoot a LOT of, deadly on game to any reasonable 45/70 range & PERFECTLY SAFE for your old carbine. He said that you should look for a .460 double mould & use the bullets UNSIZED & simply dip them on ALOX for lubricant."
fwiw, i SHOULD have asked Ed FIRST, before opening my pie-hole.
(Ed has forgotten MORE than most people know about reloading.)
SORRY for the trashy advice.