AA5744 is a good choice, my personal favorite is IMR4198... it works well in the .45-70 and in other cartridges as well. As a rule, I try to keep my .45-70 pressures below 15K PSI, which is easily done with either AA5744 or IMR4198, I have some QuickLoad data run off by a friend that supports this, if you are interested.
2400 and Unique are another avenue to low-pressure plinking loads. The standard load is 15grn Unique under a 405'ish cast bullet for around 1200fps. A word of caution, however... that small charge of Unique can be position sensitive in the big .45-70 case, I know of one shooter that had that load blow the case head out and he almost lost his eye. (This is factual... I was there when it happened, not 3rd person.) There is a theory that Unique needs to be set against the primer for consistent ignition... a tuft of Dacron does that. Do NOT use cardboard or any other type filler. I have loaded and shot that 15grn Unique load in my 1885, I 'muzzle upped' my rifle after chambering the round to make sure the powder was against the primer before firing, but it's likely I will not continue with Unique... IMR4198 does everything I want, and without problems.