I fit my first barrel last week and I am very happy with how it turned out. I had a 12 gauge barrel I cut to 18 1/2" inches laying around and an SB1 frame I picked up cheap. Just for fun I tried the barrel and it didn't come close to fitting. I read everything I could find on the subject and gave it a try.
The barrel would only close about halfway so I worked on the hinge area first. I started with a drill bit wrapped with sandpaper but that was slow going so I used a round file and then when I got it close switched back to the drill bit. Eventually it would close but not latch so I worked on the shelf until it would. The whole process took about 45 minutes.
Go slow, take your time, check for fit often. When you think you've got it close put the forend on and try it, on mine that seemed to pull the barrel forward a little and that affected the fit. Better to leave it a little tight and let it wear in than have a loose, sloppy fit.
You can do it! Good luck.