Your Marlin 1895 Guide Gun .45-70 loads are perfectly safe in your .45-70 T-C Encore. Remember, the Encore is chambered for many high intensity medium caliber rifle rounds, so the warmish or hot (for Marlin) loads are okay. You may have to work up new accuracy loads for it.
As for what is the effective range with the longer barrel? That is limited by trajectory of the .45-70 AND your shooting skill, more than anything else. For most shooters using iron sights, the .45-70 is a 200-225 yard deer gun. Sure, competitive shooters can easily stretch that to twice the distance, but but they shoot all the time at known distances and windage. Your Encore effective range is more than with the guide gun, but not much more.
