Go in between to 38-55. You can also shoot.375 Winchester if it is a newer action and strong enough..It will do everything the 45-70 will with less recoil..
I think you mean 38-56, unless you are planning a total rework of the internals.
For the OP - if you already have enough rifles in 45/70, then a rebarrel in 33 Win or go big and build a 50/110.
A fast twist 45/90 is another option, but in reality in an 86, using bore riding cast designs, you can load a 45/70 to 45/90 length and performance. The 45/70 in an 1886 is pretty hard to beat for versatility, component and load data availability.
