I load the Flat points 350's over I believe 42grs of 3031 cant remember off the top of my head since ive about switched to all hard cast. It shot well in all my rifles and recoil felt about like factory 300grainers. I make it a point not to load any of my 45/70's or 45 colts above trapdoor and SAA levels in case they somehow find their way into another gun. I just think out to their normal ranges, it really isn't going to make that much difference going any faster. I load 405 casts over 38gr of 3031 and my guide gun will bughole them at 100. My sharps shoots them equally as well if I do my part. They perform very well on game too and a lot cheaper than jacketed bullets
I think the 350 grain Hornady round nose or flat nose is a good bullet for any thing in North America and I have shot them quite a bit. I have used the 405 grain super premium bonded flat nose from Alaska Bullet Works here in Alaska because of our big bears. I have about 80 of them loaded in WW cases and when they are gone I am loading up a 350 grain version of the same bullet in new Starline brass.
I have carried and shot a Marlin 45-70 since 1990 and never killed a critter with it. My customized peep sighted and octagon barreled Marlin holds 7 rounds and is comforting to have around for our Alaskan outings. I tried IMR 3031 and RL7 and long ago switched to H322 and WLR primers for all my 45-70 loads. It fills up the case and the bullet sits on the powder and with a good crimp they stay put.
Really hard to come up with a bad bullet for killing with a 45-70 and even a 300 grain hollow point is gonna hurt what it hits.