Hey Mik1
I don't have any experience with the 40-70BN but I have been shooting a 40-2.5SS for about 5 years and a 43 Mauser, which is a BN case, for about 7 years. I'm not certain what I can tell you will be applicable but I'll tell you what I do know about the 40 and BN cases.
All I've ever shot in the 40 is GOEX Ctg. except for about 1/2 lb of T-7 for learning purposes. I've never seen Swiss for sale locally but I'd sure like to try it in three rifles I own. I've always shot the Lyman 410 gr. bullet in my 40 and the results have been so good there was no reason to look any further. I've used that bullet for hunting and it's performance has been all that could be asked for. I've also used it to 500 yards and it was very, very good there. It shoots better than my old Long Range Express in 45-70, and that's saying something. The powder charge has varied from 60 grains, where I started to 62 grs. which is what I now use. You should probably check the twist in your rifle and if it's the slower 1-20 it may not stabilize the long 410 gr. bullet as well as the shorter 330-370 grainers. The twist in my rifle is 1-16.
All the bullet needs to do is hold the powder in place. Compression of approximately 1/8 in. has worked very well for me for many years in several different rifles. You can seat the bullet out as far as is practicable. You can measure the OAL of the loaded cartridge which will determine your powder charge. I've never needed or applied heavy crimps to any of my BP cartridges.
The 43 Mauser is the only BP bottleneck cartridge I ever worked with and I never found it any more difficult to load than the straight cases. It has been said and written that BN cases foul more than straight cases but I never found that to be true with the Mauser. As with most BP, BN cases the Mauser has a long neck and my load for it comes a little more than half way up the case neck. As I stated above, your bullet length and seating depth will determine your powder charge, unless you choose a light load and use a case filler of some kind, a practice I've never found necessary.......with the exception of a Martini-Henry chambered in 577-450. That is a huge, BN case I worked up a load for, for a friend, several years ago. So I have expereince with two BN cases, I forgot....hehehe.
The rifle I own chambered in 43 Mauser is a German, double rifle manufactured in the late 1870's-early 1880's so hair splitting accuracy wasn't in the realm of possibility. I did finally get it to group around 3 inches at 50 yards with both barrels altho each individual barrel shoots around 2 inches. That's just double rifles.
I don't know if you can extrapolate any useful information applicable to your rifle and cartridge, but the above is what I shoot in my 40-2.5(70) and my experience with BN cases. Good luck!
Vic
