You should trim the cases for uniformity. I have noticed that some of my brass actually grows shorter from firing. Guide Guns do like lead boolits and they like them sized fat. I size mine to .460 and have no leading probs to speak of.
The recoil of a 45/70 can be reasonable compared to 12 Ga 2-3/4" slugs. It can easily be loaded even stouter but I have not found loads of that level pleasant or needed for any animal up to and including Elk.
Reloading the 45/70 is a breeze and can loosely be compared to loading a 44 Mag or 45 Colt, figuratively speaking. Be careful reloading the 45/70, if you drop a case, it will need resized, if you drop a boolit, its almost always ruined with a big dent in the base or nose. My GG likes Reloader 7 powder with Cast lead slugs. 3031 gave good results also. Be warned not to buy lead bullet cartridges for your GG. Most are .457 for the old guns and will leave your barrel leaded beyond frustration.