I used Trapdoor Springfield smokeless loads in my 71/76 with H4895 powder to good effect, after re-chambering. However, with the 77gr. original black powder charge with 370gr. bullet, capacity wise, the light .45/70 loads should be just fine in that rifle as well, Curly.
One could start lower, of course, using backer rod if necessary.
Of course, BP could also be used with full sized bullets.
That bore will clean up easily.