I will add too what is all ready here.
I use regular RCBS Dies, and a Lyman 310 tool to load 45-70's. However the hardest lead I have shot in years has been aprox 20-1 to ....uhm dead soft 40-1 and nothing other than real Black powder (FFg and some FF/ FFF 777 when I couldn't get real black, for those looking for real black try powderinc )
For those unfamiluar with the 310 tool, it does it all , except full leangth re-size. I use it on Remington cases to get a full charge of powder in the case, makes them look a little more bottle necked, however this method kinda makes it gun specific. ie rounds form the rolling block didn't work in the guide gun, or 86 Win, or my budies trapdoor...
These dies are for cast bullets and work very well with blackpowder loads, but can be very aggressive on the flare and ruin cases, as can the crimp. I personally like the crimp these dies provide.
***However the agressive crimp can ruin cases if you do not have your cases trimmed to the same leangth. This allso is important for accuracy beyond 100 yds, as case leangth will effect the amount of crimp aplied round to round, which will effect velocities and create up and down stringing.
I use a standard set of RCBS 45-70 dies but to get an adequate flare on the case I have to set everything down as far as I can get it to get a good flare.
These dies are full leangth resize, they run about $35 $45 bucks per 3 die set and have searved me very well. I have seen no need to pay the extra for a set of RCBS Cowboy dies. I do have a set in 32-40, and they are very much like Lyman dies, I have 2 sets of those, 1 in 38-55 and a set in 45-90. Lyman dies are about the same as the RCBS but seem to be more BP shooter friendly, and I do prefer any of the dies I mentioned over the Lee dies (I own some of them too, .9mm and .223, I didn't care for the .45 dies I had from them) The standard RCBS or Lyman 45-70 dies will work fine and all truth be told current green box RCBS dies are designed around 45-70 lever guns with hard cast bullets and smokeless loads.
as I mentioned above I no longer shoot hard cast, and cast my own bullets these days. I do give warning here if anyone is interested/ new to casting...wheel weights today are not what they were 20 years ago and are too hard. Look for sheet lead and tin, Roto Mettles sells tin and dead soft lead, or you can buy it pre mixed. 20-1 lead would be very good option for casting your own, and BP loads. The crimp is very important for long range accuracy and allows the soft bullet to grow slightly creating a a gass seal. Hard casts used with BP loads will not grow, and allow blow by which leads the boore....I allmost have out of my Rem... On smokless loads the crimp is less important in this aspect and mainly holds the bullet to keep it from sliding back into the cassing under recoil however that can create accuracy and pressure problems.
So here is a simple way to decide to crimp...1. it loads into a tube, or 2. was made prior to 1900 it was likley sedigned for BP and those were crimped, 3. wonder magnum hand cannon (500 S&W) Crimp it
As for a good load, I will suggest the Lyman reloading manuale, and suggest that you do not exceed 450 grn bullets or you may have feeding issues due to weight and overall cartridge leangth.
Hope this helps answer some of the upcomming questions about 45-70 here on the thread that was started.