If I understand correctly then there are differances in the SB2 shotgun and the rifle??You should get the SB2 shotgun frame as it has bigger firing pin.
I got this 45-70 with a trade on some other stuff i have only shot i a few times and i was thinking of ordering a shot gun and a light small calibur rifle barrel and make a trunk gun / just to bring along gun. but it would also be interesting to build a SG FH barrel for it along with spare weigted stock in case polar bears move into wisconsin due to the recent US ban on bringing back hides from Canadia.
after spending 4 years develping the ultimate deer hunting AK pistol for the local shotgun/pistol hunting area. maybe I do need a SGFH to go with it.
I was thinking that a real long 20 or 28 guage with a rifled berrel might give better FPS an accuracy and not have the recoil. just thinking out loud here