I'm sorta a one gun one bullet and load guy, so read my advise with that in mind.
I have a lot of customers who compete out to 600 yards with the 45-70 and black powder. My best bullet will be the SP1R, a one inch radius spitzer, at about 500 grains. Best alloy will be sweetened WW, water dropped, and best diameter will be whatever it takes to get your chamber full when you load the rounds. Black has to be run compressed, so use just whatever load that is. Velocity will be a bit sluggish, but you'll get a lot of sideways glances when others down the line watch your groups form.
Don't use any of the LBT lubes for real black powder. Any of the brands which say wonder lube are good, and all are made by one company. I hear lightening lube beats it, but last I knew it had to be purchased in 10 pound lots. The black powder gang can fill you in on this, and I hear the rage is to split and order with several other shooters. If you shoot a substitute powder, LBT lubes can't be beat.
If you think you'll be hampered by the arching trajectory of a 1200 fps bullet, they rechamber for the longer cartridge, but shoot the same bullet. I exect the extra recoil and powder cost might dampen your interest in the larger cartridge though. Probably the best way to flatten the trajectory would be to go with 450 gr in the same nose profile. The extra powder space will hurry your bullets quite a bit.
Read down through all the 45-70 posts and you'll pick up a lot of helpfull information, I believe.