OK- found some loads to get you going. Since Mike noted the factory load being 55gr. BP and a 380gr. bullets, these should give you an idea. Case used below were 1.8" and 1.90" long, being a shortened .45/70 straight sided hull, all primers, standard large rifle.

: 345gr. Cast bullet (#457122 HP Lyman) 25:1 tin/lead, SPG lube - all bands in the case 60.0gr. 2F GOEX - av. 1,064.4fps.

: 303gr. Buffalo Arms mould FN - 14.0gr. Trail Boss Av. 1,202fps 1.325" @ 50yards tang sight and blade.

: 303gr. Buffalo Arms mould FN - 37.0gr. IMR3031 Average Vel. 1,261fps. mild easy shooting 1.5" 50 yards Winch. open sights;

:303gr. Buffalo Arms mould FN - 13.0gr. Herco Foam backer rod 1/2" - 1,180fps Av. 1.2" 50yards tang sight and blade.

These pressure tested loads were loads I found elsewhere and had recorded for my brother.

You might google Barnsness .45/60 loads. He did an article some time ago about this round in M1876 Rifles.

385gr. cast presume Lyman mould - might have been gas checked.
:Varget ---- 42.5gr. - 1,537fps - 15,400CUP
:H4895------ 35.0gr. - 1,280fps - 11,900CUP
:H-Bch.Mark- 50.0gr. - 1,621fps - 16,300CUP
:IMR4064 --- 47.0gr. - 1,545fps - 18,800CUP
:Trail Boss- 14.0gr. - 1,075fps - 19,400CUP

Note the actual pressure developed by TB - compared to the others. Just because it produces low velocities, does not mean it produces sub-normal pressures.

Hodgdon's own data shows it produces up to 25,000PSI in the .45/70 with heavier bullets and maximum to the base of the bullet's loads, even though they feel like very slow, low pressure soft-ball loads. Slow they might be, but the pressure is up near max for many weak actions.
Bottle-necked ctgs. can produce well over 30,000psi with jacketed bullets, but of course, this is well below the normal 65,000psi some ctg.s produce with FP loads.

I would be VERY tempted to start with either IMR3031 or H4895 as both work well with lower pressures in the 20,000psi range, depending on bullet weight. H4895 could be reduced to about 35.0gr. to start - chrono and go from there, using a 380gr. bullet.