Thought about this before...
I understand what you're asking, because I'm in the same situation and have done what you describe.
It can be made to work, but several obstacles present themselves.
1. If you manage to do it, frequent cleaning of the chamber becomes paramount. You are creating the same scenario as shooting 22 shorts in a 22LR, 38 Special in 357 Magnum, etc. Powder fouling in the chamber becomes more apparent with the shorter case and will rear its ugly head when you switch back to the longer cartridge. Not a huge problem, and it is easily dealt with. But something to remember as the more you let the fouling build up, the harder it becomes to remove.
2. Based upon my experience with the various brass I used, the brass may need considerable modification besides simply trimming it shorter.
The further down towards the body of the case you go, the thicker the brass as a general rule. You are now trying to take the original body and turn it into a neck, and now the bullet is seated deeper than the brass manufacturer intended. Depending upon final dimensions of your bullet, it is possible to experience considerable bulging of the brass when seating the bullet, which may lead to chambering problems. Due to the increased thickness, the brass must now be outside-neck-turned (best) or inside-reamed (still acceptable) to ensure reliable chambering. The need to do this will depend upon the thickness of your particular brass and the chamber tolerances of your gun.
Also, remember that the further down towards the body you go, the harder the brass. How hard and where that hardness begins will vary slightly from one manufacturer to another, and sometimes even between different lots from the same manufacturer. The new neck location is now in an area originally designed to be harder. Annealing is necessary to bring the neck to proper softness so as to facilitate crimping and firing without splitting the case mouth/neck upon (or even before) the first use.
If you do try this, your best results will probably be with Winchester cases. BP shooters love it because of slightly more internal case capacity, meaning thinner walls that may help avoid the requirement to neck turn as mentioned above.
In conclusion, the process is feasible, but usually requires more effort than what most are willing to expend. Once you have dealt with the brass, then you get to figure out crimp locations or even design a long-nosed bullet. After that, you scratch your head trying to determine the proper way to develop a safe load, since the standard 45-70 load data will no longer apply.
In the end, you will probably do as I did and say "OK, where's the 1894?"
But it is nice when you are carrying your guide gun and get stopped by the game warden, then he gets mad when your barrel says 45-70, yet you are still LEGAL.[smilie=1:
