Ullr,
In Pet Loads from Ken Waters, he shows the following loads for the 45-70 for use in Marlin, Ruger and Siamese Mausers:
43 grains IMR 4198 with 300 grain Hornady = 1882 fps
45 grains IMR 4198 = 1932
50 grains IMR 4198= 2057
All are from 22 inch barrel, of course you'll get less from you 12 inches of T/C.
Those 300 grain Hornadys are fine bullets and will kill anything that walks on this continent, might not be my first choice, but they will kill and kill cleanly. They have also been extremely accurate in my Ruger and Marlin.