Yes, Marrakai - I agree that the headspace should be checked.

Is this straight case 2.4". 2.6" or 2 7/8"? All were loaded with 100gr. of powder with certain bullet weights. The 2.4"Sharps also called the .45/90 by Winchester was loaded with up to 100gr.BP and a 550gr. bullet by Sharps.

I would hold loads with the 350gr. to 2,250fps in a 2.4" case, but a longer case should get a bit more with no stretching. Stretching is usually caused by headspace problems, not due to maximum loads. 2,250fps can be achieve in a .45 2.1" in a lever gun at 43,000psi. The longer cases will deliver the same velocity at slightly lower pressure & this depends on the case length. A 3 1/4" .45 case will deliver 300gr. bullets at something like 2,400fps to 2,600fps at 34,000PSI using H4895 (or maybe IMR4895) This was Lyman data, can't remember off-hand.

The Speer 350 gr. should make a terrific North American big game round at 2,250fps - for all heavy game. I'd not try for higher speeds in that double.