Sorry, my bad. I thought you were talking about using 45-70 bullets in a muzzle loader (ML forum, ya` know
).
Both Harvester and MMP make .45 caliber sabots for .40 caliber bullets, and MMP makes a .45 caliber sabot for .355/.357 bullets. I suppose they would work, though I've never heard of them being used that way. I have a 45-70 Browning 1885 and can't think of any reason to try to improve on the load I'm shooting (300 grain Rem SJHP @ 2000 fps with 58 grains IMR 3031 - shoots 1" @ 100 yards and under 3" @ 200).
You might check the black powder cartridge rifle forum and big bore rifle forum.