H&R will tell you that, but they chamber the Handi in 500S&W with a larger case head with operating pressures well over 50kpsi, the inside case head diameter is .501", that calculates to over 9800lbs of breech thrust. Starline 45-70 brass has an inside case head diameter of ~.420", that calculates to 6900lbs of breech thrust at 50kpsi..
If the 500S&W is safe, so is Ruger 45-70 data, but beware of the the recoil associated with those loads. Most shooters don't like the recoil of even levergun loads, so they stay in the trapdoor level or low levergun. 45-70 pressure levels vary quite a bit, but Hodgdon and Hornady both set the levergun limit to 40kcup, Ruger at 50kcup, Barnes goes to 55kpsi. Take a look at what firearms Buffalo Bore ammo is intended for.
Tim
http://www.riflebarrels.com/articles/custom_actions/bolt_lug_strength.htmhttp://www.buffalobore.com/ammunition/default.htm#4570