It is advisable not to shoot .45 Colt ammo in a .45/70. Steve has the measurements. Most .45 Colt ammo is about .476 to .478", while the chamber of most .45/70's is .508" to .510"(sometimes more). So, we can have more than .030" (almost 1/32") smaller cae being fired. A heavy .45 colt load would surely burst, while light gallery type loads would expand radically. There is no reason to shoot .45 Colt ammo from a .45/70. If you want a light load, develope one using .45 Colt bullets - I did.
: Using .452", 230 or 225gr. swaged Speer bullets and 15gr. of AL8 pistol powder, I achieved a velocity of 1,320fps and 3" accuracy at 100 meters. At 25 yards they printed on the heavy plex of the 2X scope (making 3/8" groups on centres) while the cross hairs were zero'd for my heavy hunting load. I shot 34 grouse one fall with this 'light' load, taking their heads off cleanly. Many other powders will work as well. Alcan's AL8 may not even be in print now, but Unique, Trail Boss and others are. Trail Boss might be the best of the bunch for loads in the 900fps to 1,300fps range. Use full length .45/70 cases and pistol primers.