The double I built is based on a French 16ga and the barrels are soldered into the origional mono block. I did braze he quarter rib in place however and all has worked out fine. The rifle is chambered for the .450#2 NE cartridge and shoots about 1.25" at 50 yards. If you are like me you will enjoy regulating the gun it was a lot of fun!!! What made you choose 45-70 in stead of a more clasic double rifle cartridge?
I posted building my rifle several years ago you may find some helpful info back there if you can find it. I am now building a 475 on a Verny Carron action(another French gun) or at least I think it will be a .475 At present I am making new forarm iron as that was missing. In any case the gun will be either a .475 or a .470. Good luck and have fun!