I can't say what will work best in your rifle, but I can share some experience. 

I shoot 45-70, PP and GG, with Black Powder, and exclusively use Olde Eynsford 1.5F. I compete with that rifle, but it's all 300 yards and less, so may not tell much. I use a lot of compression, much more than conventional, and have low extreme spreads and an SD of 11, which is good enough for me. 

I recently got into a Mauserbuchsen, single shot, in 9.5x47R, .375" grooves. And I tested with OE 2 Ffg and 1.5 F. Despite the 360 bore, this rifle didn't want to shoot 2Ffg at all. It was remarkable how much better light compression 1.5F was with both PP and greasers. The difference between groups and patterns.

So, this sort of flies in the face of conventional wisdom that suggests 2Ffg starts to deliver improvements at .40 cal. If you were getting 2MOA groups with 1.5F, I see no reason to change. As to Swiss vs OE, I have used both and I am not a good enough shooter, or do not shoot at long enough range to see a clear advantage for Swiss, but folks that shoot better and longer than I do swear by Swiss.

In other words, clear as mud. 

Long time ago a old shooter told me to stay away from the 40-70. Said they're finicky about what grade BP they liked. Mighty glad I don't have one.

Though 1F it is not available except cannon powder, Curtis and Harvey loading data lists it for 45 caliber reloads ... (You need to Login or Register to view media files and links)

I'm not implying that the .40-70 BN will be the same as .40-70 SS. But in my .40-70BN, Goex 2F worked quite well.

reading Freunds book. he loved the 40's and a lot were 40 70 ss.
  he recommended 2f way back in the day.

    I use 1 1/2 Swiss in mine.  measures great and compress to about 1/8 or less.

westerner wrote on Jun 11^th, 2020 at 10:13pm:

In the years I've been shooting BPCR/BPTR (10+) I've known half-a-dozen decent shooters who have tried the 40-70SS. Not one of them has stuck with the cartridge even though a couple tried for as long as three years. 

Only person I've ever heard of who won a few matches with one is Mike Venturino back when BPCR first started and scores were considerably lower. Once people started to learn, the 45-70 and 40-65 pretty much took over and are, by far, the most popular and easy to get to shoot well to this day. A couple of good 38cal shooters and a couple of good 45-90 shooters, but 80% of folks at the nationals will be shooting either the 45-70 or 40-65 for good reason.