Hey Bill,
As you know, I shoot my original 76's on a regular basis. For the .45-75 and .50 Express, I have never used anything other than reformed .348 Winchester brass. I have never had the problem you're experiencing with this brass. I do not anneal this brass down past the shoulder, but I do know it is thick and tough as nails. In your case, annealing down half way to the web near the base, might be the answer. Also have to mention that I never use magnum primers, only standard Federal large rifle. Keep us posted and good luck.