Siamese does OK with the .348 case, as the rim is the same size as the .45/70 - just less difference from body head to rim diameter. A .450 Alaskan or even .50 Alaskan would also be good choices.

Starline even makes .50 Alaskan brass marked cases now. My bro just received 200 of them for his Sharps. He uses it as a .50 2 1/10" BP round with 91gr. 1 1/2 Swiss with a 600gr. Buffalo Arms bullet.(mould) It's an excellent shooting rifle so far, to 600yards. The bl. is from Harry McGowen (St. Anne. Illinois), 24" twist.

Moulds run down to 300gr. on the light side. A .50 Alaskan easily gets over 2,000fps developing quite moderate pressures with all bullet weights. 600's might struggle to get much over 2,000fps. though, depending on the throat.