It will fit but it requires diffrent tools and yes you should load black by volume. And I wouldn't use FFFG I would use FFG. In order to get 70 grains of black into a .45-70 case you have to use a drop tube, Which is a long brass, copper or alum tube case size bout two foot or so long with a funnel on top, you pour the powder thru the tube that helps settle it in the case you will still probably need to compress the powder(as it should be with black) to seat the bullet. Triple seven is not the same as BP or any other sub I do believe it has diffrent loads to compare with true black but I am not 100% sure what.