There are no carbide dies that I know of for the 45/70.
The 45/70 is not a thru straight wall like 357 mag there is .012 taper per side or .024 total .
Not a lot ,but enough to rule out carbide die.

Yes you have to trim the cases ,You should be trimmimg the 44 mag cases if you want accurey in moderate to hot loads ,depending on the load my 44 case need to be trimed every reload.

As for powder ,what pressure level are you going to load to ( what gun ) Trap door, Marlin or Ruger #1 ??
What bullet cast or jacketed ??
For cast bullet and trap door loads AA 5744 is hard to beat
Also RL-7
IMR 3031

For Marlin loads
RL-7
IMR 3031
IMR 4198

Hope this can help

Johnch