Quisto, I don't profess to know what the "optimum" barrel length is for a .45-70, but here's what I spec'ed when we were building my roller last year: barrel-Green Mountain, length 32", taper #3 1/2.  Mine is built on an original No. 1 blackpowder action, straight grip, with steel nosecap and buttplate.  I still had to add 1 pound of weight to the butt to get it up to 12 pounds, with sights. 

The 32" barrel I like for the increased sight radius...whether it makes any difference to whether all the powder is burned or not, I don't know or particularly care...just that it shoots good.  What I DO like and care about is that the heavier breech section of the #3 1/2 taper, plus the lead in the butt, moves the balance point of the rifle to right about where the forend screw is...in other words, it isn't terribly nose heavy like a similar rifle with the #4 taper and no added weight.

Greg