Whoever said it is "like asking best load for a 30-06 there are bizillions of choices" was right. What he was wrong about is, that doesn't mean there aren't powder and bullet combinations that consistently work better. 4064 and a 155 grain Sierra, for service rifle competition in 308 in a known winner. If your 308 won't shoot this combo, get a new barrel. Benchmark for 223 target rounds, same deal. I have 6 different 22 Hornets that all shoot 12.6 grains of Lil"gun and a 40 grain BlitzKing bullet well under an inch. Best one 1/2" at 200 yards, worst one .65 at 100. No other powder will do that in a Hornet.
I have shot thousands of 45-70 rounds. Have tried every powder suitable to the round. For full bore hunting loads in the 300-400 grain weight, 3031 has worked great in many different guns. Second most favourite, and best one for the 300 grain bullets is 4198. You will pick up 200 FPS over 3031 in the 300 grain bullet and they are pretty much equal in the 350 and 400 grain bullets. My go to powder when working up loads for the 45-70, which is not at all picky to load for, is 3031. If it won't group under 1.5" at 100 yards with somewhere around 58 grains of 3031 and a 350 grain Hornady RN, get the gun fixed. Most of the guns I have used it in (make sure they are modern levers or single shots, not old trapdoors) will group between 1/2 and 3/4 of an inch with 350 grain Hornady round nose bullets.
Let us know how you make out.