Modern .45/70 case have been stress tested to 70,000psi and litterally dropped from the chamber. This info coming from Brian Pierce in his article on the .45/70. Starline was the brass used, I assume from the article. The strength, ie: head dimensions, etc, will be the same on your 2.6" brass. Be careful on brass neck thickness, though as it may be a bit thick, if shortened from 3 1/4" brass. You may have to turn cases, maybe not.
: SharpsNitro - I'd have no problems whatsoever with your project. l've loaded .45/70's in both Siamese Mausers and Ruger #1's to the same velocities as my .458 2", ie; 400gr. at 2,200fps and 510gr. Solids to 2,060fps with no outward pressure indications. The 2,060fps matches .458 WinMag velocities actually chronographed by Speer in a Ruger, way back in the 70's ro 80's. Obviously, the .458 was being downloaded, even at that time. You should easily make 2,150fps in your 2.6" case - probably 2,200fps with today's more modern powders.