I shoot .38-55 instead of .45-70 but it's still a long straight walled case with a relatively heavy cast bullet.
I've got and tried 4227 (likely not good for the bigger .45-70), 3031, AA5744 and 4198. I've found that I like the 4227 and 4198 best of the bunch.
One aspect that I found odd is with the AA5744. It was a .45-70 shooter that put me onto this powder. HIS results with the bigger bore and heavier bullet was that it burned clean and produced next to no fouling in the bore. I didn't look at the brass. In my case with my 265gn bullets I found that I had a lot of unburnt granules of powder laying in the bore. I figure I was just not generating a high enough pressure to get a clean and complete burn. But looking down that guy's .45-70 barrel was enough to make me buy a pound of the stuff on the way home! I should take it along to my cowboy action meet and sell it to one of the big bore guys. There's also LOTS of success stories with AA5744 used in .45-70 on the web.
The 4227 worked pretty good for my use but it's not a listed option near as I can see for the .45-70.
I have not had a chance to play with the 3031 yet.
The H4198 seems to work very nicely. But the volume of the charges used leaves a BIG lot of air space. Reading about these loads on another forum I found a reference to using a small loose tuft of polyester pillow packing to aid in holding the powder in place. Initial testing suggests that I'm getting a little better level of velocity consistency vs with no tuft. But I need to get out on a calm day for some serious testing.
There appears to be some difference between H4198 and IMR4198. I ended up with both without realizing that there was a difference. How much of one remains to be seen but there are a few cases of loading data on the Hodgdon reloading web site that list both separately and which show slightly different peak pressures for each. So watch out for that. But then some say that they are the same. For now I trust the Hodgdon web site.
Keep in mind that I'm shooting single shot rifles and the cartridges are stored and transported bullet upwards. The bullets only get laid on their sides when placed on the bench or stool (field shooting from a second stool with crossed sticks) and when put into the chamber. So there's little or no tendency for the powder to "filter" through the filler. I would not use this method on a repeating rifle as the recoil would likely cause the powder granules to work their way through the strands of the filler. If this would be enough of an issue on a two shot SxS I don't know. It may be worth playing with other granulated fillers like cream of wheat or oat bran or see for your case if it matters that the powder is loose. Perhaps leave it loose and before each shot incline the gun upwards to around 60 degrees then bring it back to level so that the charge has the same packing location for every shot?
For what it's worth you can try to find a lighter load. But if it's shooting well at the 1550 with that bullet it may well be that you need to keep pushing at the same speed even with the new powders to maintain the accuracy. Only testing will tell.
As for clean bores the 4198 does foul the bore in my case. But not heavily. Looking down the bore after a session shows clear signs of rifling that looks as if it was painted with a light coating of flat black BBQ paint. The fouling cleans away in a couple of solvent patches without any issue. With your heavier bullets you may not even have that much fouling left. The cases are black inside but clean up to a nice bright brass colour when decapped and cleaned for 15 minutes in my small ultrasonic cleaner using a 3:1 mix of water and cheap white vinegar with a little dash of liquid laundry detergent added. And again that's with a rather light bullet and low peak pressure. In your case I'm thinking it would produce a bit less blackness.