looking at a 45-70, but what ammo?

im considering buying a 45-70 handi.
how do I know what ammo is safe to shoot?
I looked at different factory loads,
some in 1500 fps range, saw some around 2000fps.
from 300 gr to 405 gr.
its on an sb2 frame. but im clueless about 45-70.
thx!
Joe.