The 40/70 is 2.4" long and the 40/60 is 2.2" long. If you fire a 40/60 in a 40/70 chamber the bullet will have a .200" jump into the rifling. If using black powder like you should be the bullet will expand some in the space between the neck of the cartridge and the chamber throat causing a loss of accuracy when it gets swaged back down into the barrel. The chamber on those Maynard cartridges is generally about .450" diameter which is quite a bit larger than the bore. You should be using a proper length cartridge in that chamber.  405 brass shortened is a good candidate if you can find some.