When I purchased my Buffalo Classic I called Marlin and asked them which loads were appropriate for the rifle. I was told that loading for lever actions was ok. Loads designed for the single shot ruger were too hot and could damage the rifle and possibly the shooter. Since I like to err on the side of caution I have stayed within those limits and have not had any problems. 
Levergun loads are "appropriate" for the SB2 frame, but modern rifle loads are safe too, if they weren't, the 500 S&W wouldn't be safe either, it's loaded to a SAAMI MAP of 60kpsi which is 5-20kpsi higher(depending on data source) than any 45-70 load and it has a larger case head and less case wall contact which increases breech thrust. But in the end, your shoulder will tell you what load is "safe".

Tim