The problem with long BP cases is they are not light-load friendly. there is too much empty space.

Some powders like Trail Boss might help, but beware, pressure with that powder will outstrip the ballistics it delivers. Even a low vel. load of 950fps in the .45/70 produces 24,000PSI. Longer case, more trail boss, more pressure and sub-standard velcoity. It is designed to fill a cae and produce pressures that are low for a very strong action, without having the difficulty of low load volume. It does not produce low pressure, per se'. Slower rifle powders do.

You can get good to terrific ballistics from the Baikal in .45/70 or .45 3-1/4" and still hold pressure down to 28,000PSI.

5744 might be the best bet for a longer case with cast bullets, if searching for a cheap, low pressure, low velocity load.