The main problem with using Bertram brass in the .43 Spanish Remington Rolling Block rifles was that the cartridge cases tended to have thin rims, and that, in combination with oversized chambers gave an excessive headspace condition. Thus, the cases tended to stretch and separate after relatively few firings.
The fireformed .348 Winchester cases work out much better, with thicker rims and sturdy case construction, and I've used them with good results in .43 Spanish, .43 Egyptian, and .50 Swedish.
The real secret in long case life is to avoid full-length resizing and setting back the shoulder. The chambers of these early RRB can be pretty sloppy, and repeatedly crunching down the cases to fit the chamber is a guarantee for case failure.
As for your question of using .45-70 or .45 Colt seating dies to seat .43 Spanish, I don't believe it would work. The .43 Reformado/.43 Spanish cartridge is larger in the base dimension than .45-70, so the case won't fit in the seating die. The "neck" is too short to reach the crimp shoulder and also will not fit in the .45 Colt seating die. Actually, for blackpowder loading, case sizing is not even necessary, nor is mouth crimping. The old single-shot rifles don't need a tight bullet-to-case fit, and the cartridge doesn't have to cycle through a repeating mechanism. After initial fireforming using pistol powders, tissue wad and Cream of Wheat, and neck expanding, they are ready to load. I normally use a grease wad, seat the bullet by hand, and load and fire. The cases I use have held up to dozens of reloads without any neck or full-length resizing.
