My working assumption is that any load the Marlin 1895 can handle, the .45-70 Handi can handle as well (and then some, maybe, but I'm not going to push the envelope). I have a load that pushes a 350 gr JRN to a little over 1900 fps with 50.5 gr of RL 7 (I call it Neptune) and it's at the top of the Marlin level. I've fired more than 100 of these loads through my Handi with no signs of any problems so I felt comfortable enough to let my brothers and friends shoot them during recent outing. Of course, they all oohed and awwed about how powerful it was and how hard it kicked.
But now I've got some H4198 and the Hodgdon manual indicates I could push the same bullet to 2190 fps and still stay under the 40,000 CUP recommended for Marlins. Ouch! Unfortunately for my shoulder, I'm prone to let curiosity get the better of me.