The words "SAFE" and "good idea" can mean very different things at different times.
Firing the regular .45-70 cartridge in the much longer chamber regularly is not a good idea. You have a very good chance of "ringing" the chamber and damaging it for firing with full-length .45-120 rounds.
In an emergency, yes, you can (and the old timers did) fire shorter rounds in the longer chambers.
