Lyman's jacketed reloading book, maybe #46 or #47, or even the cast bullet handbook (large one) has data on the .45 3-1/4"(I assume that's the .45/120 you are talking about) to 35,000psi.
: If you are referring to the real Sharps .45-120, (.45 2-7/8"), then you are pretty much on your own to develop some safe ammo for your double. The only data I know of, is limited to 28,000psi or less.
: The 3-1/4" brass is suseptable to stretching with heavy loads. The 2-7/8" brass less so, etc.