John/Nitrox,
Before I bought this rifle from the widow of the maker, I collected the history, Beretta factory specifications including pressure testing results, and info like barrel maker and such.
I also followed the structural planning and details as directed in W Ellis Browns book.
My research indicated 45-70 pressures including back pressure on the receiver were well below the specs of the action. The rifle had been hunted for several years and was in as-new condition.

After firing it and taking a 300 pound hog with 300 grain Remington bullets loaded to 1600 fps, I decided to move forward with my planned 45-90 project.

Not wanting to take unnecessary risks, I am using VV N133 powder which generates an average of 12,000 psi less peak chamber pressure than most other powders. So far, I have kept the peak chamber pressure under 40,000 psi and it shoots very well.

I have loaded 45-70 ammo with N133 to a book velocity of 2037 fps and book pressure of 30,450 psi it too is accurate, but shoots a bit lower than the 45-90 loads. Based on my previous experience with my .405 DR, I may be able to find a 45-70 load that also regulates.

Off to a good start and much sport ahead!