Have you tried to put the barrels on your frame, as there is a good chance (assuming both sets are for an FW frame, they will fit almost perfectly. Your issue afterwards may be the forend. I had a similar issue with a 12 bore. The barrels fit as well as the originals, but the forend was a problem. Perhaps our resident gunsmith will chime in, but I had a similar issue years ago and called gunsmith Buck Hamlin who advised me to remove the cocking plate from the fore iron; then attach the forend. I did and found that the tip of the J-spring was a bit too long; so I took a file and carefully removed bits of metal from the J-Spring until the forend smapped on perfectly. I then reattached the cocking plate, and the gun has functioned perfectly since. With the replacement barrels in place one would never know they are not original to the gun without checking the serial number to the frame. Best of luck