Gil,
Your current problem of the rib lug notwithstanding, you will also run into some more technical issues with putting a new barrel on the gun. The M12 has a part labeled as a "barrel chamber ring" (#12 in the Numrich illustration) that is match reamed to the barrel chamber after the barrel is installed on the receiver. The reamer is a pull-through type arrangement, where the drive rod is inserted from the muzzle and the reamer locks onto it on the breach end. This alignment is critical to the function. A handful of years ago, I had a M12 refinished and this part was removed during the process and somehow either a part from another gun was installed or the original was clocked differently. The results were a non-functioning gun after the first shot. My advice on this particular work would be to have Simmons do it.
https://www.gunpartscorp.com/gun-manufacturer/winchester/shotguns-win/12-win
