Will be reloading some .45/70 soon, and was looking in my most recent Nosler manual at their .458 cal 300 grainer Partition flat-nose, flat base bullet.
I don't recall the exact velocity, but around 2100 fps (I think!). Certainly over 2K .....
The fellow who wrote about the .45/70 in that 6th ed manual told about a Cape Buff he killed on the Dark Continent with an 18 1/2" Marlin Guide gun.
His first shot at an old very large [and normally solitary]bull went all the way through its shoulders, and hit a cow 6 or 8 feet behind they didn't see. (the hunter and his PH tracked that bull more than 10 miles before it joined the herd .....)
He also shot it a 2nd and 3rd time.
Last shot was a Texas Heart shot that entered just under its tail.
The 300 grain Parttion that was recovered was just under the hide in the chest area.
All the advice shared about the effectiveness of the .45/70 sounds good to me!
Only thing I'd check on the lead cast slugs is that they're water quenched if I was casting them (out of wheel weights),
or if they're hard enough if store bought.
Otherwise ...... please take lots of pics of your hunt! Congratulations!