viv - That's level 3 as described by Ken Waters? The third level in most books are for modern & stronger actions like the Siamese Mauser bolt gun and Ruger #1's.
I can easily see it handling loads up to 43,500PSI, maybe even 50,000PSI, but would restrict it to those.
Those pressures will easily make 2,100 to 2,150fps with 350gr. bullets using the appropriate powders. At these speeds, the 350gr. Speer bullet would be super for anything in North America and many African animals, with it's 'over' 1/16" thick jacket.

The original .45/70 with 405gr. and 500gr. cast bullets at 1,320fps and 1,260fps repectively were good killers of bison - hostorical letters to Sharps, Remington and Ballard tell that story. It was outclassed on buffalo by the .50/70, .50/90 as well as the other longer cased .40's and .45's. The original BP loads were considered capable with the 500gr. issue loads to from 300 to 600yards. They had incredible penetration, boring a good sized hole while doing it. We have put similar loads through 3' diameter Spruce trees - it is impressive to see. No, I haven't shot any buffalo with it or any other round. Were I to get a tag, I would probably use my .45 1.9" Sharps with a 525gr. RN, load at about 1,500fps - about the same as a .45 2-7/8", the longest .45 Sharps ever chambered. That was the .45 120.