The Williams peep mounts in two of the scope mount holes. I don't have a 45-70 but I have the WRGS peep on my 22 hornet. I did have to raise the front sight. I used a front sight insert from a HiViz set. Used a finger nail file to fit it to the front sight and then glued it on. Someone suggested to me I could also use a piece of weedeater line.
Oh, if you want to change front sights make sure you read the FAQ. The front sights often have some kind of factory adhesive on the screw and you have to heat them up or you'll strip the screw head. One reason I have the peep on the Hornet is the front sight screw head is stripped.