Not an expert on anything - I can read a loading manual though, and tend to keep fairly well up to date on them as well.

A 7 pound rifle, for me, would kick more than quite enough with a 400gr. bullet making 2,000fps or a 350gr. at 2,150fps.

I would not need 'more' of that fun from a longer case. A 3 1/4" .45 case has a huge COOL quotient. The longer cases, when loaded to the lower pressures and needing case fillers can become more difficult to load with smokeless powders. Dan Pharris can attest to this from his experience with the long chamberings in Shihlo Sharps rifles and the rung chambers that resulted from improper loading techniques.

A longer chambering would deliver higher speeds at the same pressure, but not enough higher to make any difference on American Game.

Hodgdon's Annual Manuals are very much worthwhile (to purchase EVERY year for their articles and new powder data) and therein will you find data you can use for this and most any other .45/70 rifle.