Unless you are fortunate enough to have a 45-70 with a very tight chamber, 457 and 458 bullets aren't going to give you top accuracy, and especially so if they are short on bearing length. I recommend shooters of all the guns with the same type of chambering, to use a bullet diameter that fills the cartridge to a close case neck/chamber fit. This minimizes bullet tip dramatically. and does wonders for accuracy. Of coarse the barrel must be straight and true inside or it can't guide the bullet, and quite smooth if you want to shoot plainbase at maximum possible velocity. Lapping with the LBT bore lap kit will fix this.
I recommend my LFN bullet for game shooting with 45-70 rifles, especially if you want to use PB and the lower velocities one is constrained to without gas checks. If you like to get kicke around, gas checked bullets can be driven suprisingly close to 458 Win velocites from the No1. If you want to load it to 'Swift' velocities, get an M bullet in 400 gr or heavier, and 2000 fps will be easy with the lighter weight. But that's over kill or anything in north america and I'm not recommending it. Only offering what's available to scratch special itches.
With a 459 barrel you are going to be AWFUL unhappy with a bullet that's not at least that large. Simplest way to determine what the chamber will handle is to measure the inside of a few cases which have been fired with stout loads. Order whatever size you get, and a sizer .001 smaller.
About 20 years ago a customer called about getting a 45-70 bullet for his Pedersolli. He had never cast and knew nothing about it, and the rifle had not been delivered yet. I made the above recommendations, and being totally ignorant, he followed my advise to the letter. In a couple days he sent me targets which he had fired in 100 deg+ weather, in Phoenix. No groups over a half inch and many were a quarter inch. He was using heavy charges of smokeless powder and LBT bullet lube. -- If you talk to a lot of 45-70 shooters you'll find that few get this type of groups, simply because they don't understand the principles of mandatory precision.