You really should not substitute jacketed load data for cast bullets, although lots of folks do so. The extra velocity and pressures of jacketed loads can cause poor accuracy and leading, not to mention poor bullet performance on game. Especially considering that you want the rifle for bear protection, I'd use a tough jacketed bullet intended for the .458 Magnum. I shot a brownie with the 350 Hornady in my M98, it worked great. Most 400-grain jacketed bullets are quite soft.
There is a lot of dispute as to what pressure a certain .45-70 rifle should take. Some say the Marlin is "good to 40K", others disagree. Below is what the most popular loading manuals show:
Data Source�.Trapdoor�Marlin�.Ruger (CUP)
Lyman����..18,000�..28,000�.40,000
Speer����...21,000�..28,000�.35,000
Hornady���..25,000�..40,000�.50,000
Hodgdon���.28,000�..40,000�.50,000
Factory��..�18,000 reported
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