Hiram,
Strictly personal opinion, but...
I think the 685 is far too heavy for the .45-70. I think would be a very specialized bullet and might be touchy to get to shoot well. I don't see it in a .45-90, either. If you look at the winning BPCR shooters, most people are shooting bullets in the 525-545 grain range, and I'd recommend you stay in that range. 1 in 18" twist stabilizes them just fine, people get 1125-1275 fps, depending on caliber and type of black powder, and they shoot into clannish little groups out to 600 yds. and further (actually work just fine out to 1000 yds.). I like the NEI 349-C design, but I really like Fred Leeth's version much better than the ones I've seen come out of the NEI aluminum moulds. I also like the Creedmoor design, cut by Leeth, Steve Brooks, or Paul Jones. If you want to stay with a factory mould, the Saeco 745 or the Lyman 457132 (Postell) or 457125 (Government) bullets work very well in most rifles.
My bias is toward 32" barrels, but the shorter barrels are definitely easier to hold for offhand shooting. A 30" is a good compromise, with 28" a good second choice.
The 1902 action is excellent for building a rifle. I have one, extensively modified (octagon top, pistol grip) in .40-70 SS, and it works very well. I recommend having your gunsmith build a new extractor from scratch, using the full 0/1" available in the action rather than modifying the spring-loading extractor, although that one will work.
Clarence
