Hodgdon Annual Mannual 2009 book has .45/120 data held to well under 28,500CUP. In this case, as well as other straight .45 cases, the numbers almost match what the actual pezio PSI comes out at. This is a nice coincidence.

Hodgdon lists bullet weights from 350gr. to 500gr. I was surprised that the velocities recorded weren't higher for such a large case. In other words, it wasn't much higher than a .45/70 loaded with the same weight cast bullets to the same pressure levels. Bad news is the powders listed do not fill the case well and squibs and hangfires may occur. One should perhaps try slower powders than Hodgdon used, in order to better fill the case.
Try Accurate Arms web site, or Lyman #47 for other data for the 3- 1/4" case.