The maximum internal area of the .45-70 is 7% greater than that of the .30-06. Hornady Ruger-Level loads push 50,000 cup, while the '06 is loaded to the same pressure. So, the .45-70 in top Ruger level loads puts 7% more thrust on the frame than a .30-06.
That said, the .500 S&W Magnum is chambered in the Handi, and with its larger area and pressure of over 50,000 psi, it puts even more thrust on the frame. Since NEF considers the Handi safe for that big cartridge, then one could assume it is fully safe for the hottest .45-70 loads. But since I own and shoot a .45-70 H&R I know that I don't want to fire any of them! :-D