Grouse, I'm not too sure about the problem. T/C's Encore .45-70 barrel and Marlin's Guide gun both have a 1:20 twist rate and they both shoot well. T/C's 45-70 pistol barrel has a 1:14 twist rate. And most of the 40s have a 1:16 twist rate.
But going thru the numbers (http://www.uslink.com/~tom1/twistrate.htm) with a .451 groove dia and MV of 1800 fps, optimum bullet length of 1.5" would be needed for a 1:20 twist rate. That's one long bullet. For a 1:28 twist rate, all else being equal, a bullet length of just over an inch would be ideal.
But according to these calculations, my Marlin Guide gun with the 1:20 twist should never be able to shoot the short stubby 300 gr Hornady bullets. But it does and shoots them well.