Andre,  My Peabody 45-70 has a steel tang inletted into the top of the grip behind the action, (as in a top tang).  The MVA Soule sight is attached to the steel plate.  Mount the base as close to the rear as possible, and I found that I had to machine a front sight riser block so the rifle would sight in dead-on at 219 yds., which is about 10-12 in. high @ 100.  Make sure the point of the comb is at least 3/4" below the centerline of the bore to comply with the NRA BPCR stock dimensions.  This older photo shows the base before it was reversed to lower the rear sight slightly.  Tim   

I used brass for the [inlet] tang but if I was to do another I would use steel--make sure to square the "tang" with the rifle before the epoxy hardens.   ledball