Ok as promised, favorite 45-70 loads
I like the Hornady JHP 300 grain bullet you picked. Nice big hole at the business end, and if you don't hit bone a not so nice big hole on the exit side of the target. I like 53.6 grains of 3031 and a standard large rifle primer with this one. It clocks about 1,750 fps and has a ton of energy at the muzzle.
I've never tried the 350 grain offering from Hornady, since this is offered for use in the 458 Win Mag, and .458 Lott I assume the jacket is thicker than I would normally like for deer. If I were going to try this bullet I would probably try for 1,800 or 1,900 fps. A max load of 3031 (56.1 grains) is right there, along with 45.3 grains of IMR 4198. (this picked from a manual as a goal, if you try these start with a load of 47.0 grains of 3031, or 35.0 grains of 4198)
The only heavier bullets I have tried were cast. Cast bullets and the 45-70 just go together. An alloy of 20:1 lead/tin is perfect for a hard hitting hammer load. A 405 grain flat nosed bullet of this alloy pushed by 52.1 grains of 3031 is a hammer! This load is not recommended for the Handi, it is a low level load for the Ruger #1. It kicks like a mule, and sends the muzzle skyward. Back off the powder to 35.5 grains of 3031 and you have a target load that's as easy on the shoulder as a .22LR.
Have fun, go ahead and buy those 13 powders, but only after you prove that IMR 3031, IMR 4198, and Reloader 7 do not give the results you want.