Alright,
I'm going to do it.  I'm cutting down my old crusty Stevens 311 to a coach gun.  I've googled many different times to get ideas how to do it.  I don't have access to a band saw (or anyone with one), so I'm using a hacksaw.  Anyway, I saw one guy's "trick" for single barrel shotguns was to use a hose clamp to make a nice guide for sawing.  I was wondering if I used a large enough one, that it would still give me a good straight guide even if it's a SxS?

Thoughts?  Tips?

Should work. Hacksaws are tough to get to cut straight. Leave an extra half inch to square off with file.

I just used masking tape and went slow. Dressed it up with a file and some cold blue when I was done and plugged the gap between the barrels with JB weld.

Alright,
Going to do the chop this week.  I just ordered the Brownells shotgun sight kit and a T handle.  Now, since this is most likely the ONLY time I'll ever drill/tap anything I don't see a lot of sense in buying a whole can of cutting fluid.  Is there anything else I can use as a substitute?

Enjoy your new barrel regulation.

Not really worried about that.  It’s not like this is a sporting clays or long range goose gun.  It’s an old beat up Stevens/Springfield with very little value.  It’s a fun project, nothing more.

Hell, last time I fired it was like 25 years ago...

One thing I will Add.   A lot of shottys are Soft soldered If you ever think of using heat be careful i once had a aow fall apart in the curing oven . If you do use jb weld get powdered brass and dust the jbweld with it makes for a nice touch

I think I paid $30 for a shop to do it for me on a single barrel.

Nice work.  I'm happier with the finished project than if I had done it.

I cut a single barrel with a pipe cutter, squared it with a shotgun barrel facing tool in a hand drill.

This one.

Another watch-out is that the soft soldered barrels are often not as well soldered behind the muzzle in the middle section.
It has happened that after being cut off the barrels and ribs began to separate when fired a few times due to the loss of support of a well soldered muzzle area.

The old way was to use a special clamp to clamp the barrels and ribs together then fill the muzzle area with soft solder to close up the cut ends and to reinforce it.

These days I'd put some cloth or cotton in the end about a good inch down and fill it with a good grade 1 hour or longer cure epoxy, or possibly a metal bearing epoxy like JB Weld.
Use Q-tips with alcohol or Acetone to degrease and get a good strong bond.
This is an easy way to decrease the chances of problems and only takes a few minutes.

Cutting a double is a lot harder then a single barrel because it's harder to get a cutting guide square with both barrels.
For singles I used tape wrapped around the barrel and if you're careful to take some time getting it right it will also work for doubles.

The trick to keeping things square is to make a shallow one or two stroke cut on the barrel(s) rotate the barrels and make another one or two stroke cut until you've got a shallow cut all the way around.
Then just keep making shallow cuts and rotating the barrel until it's cut through.
By doing it this way you avoid getting an off-square barrel.
This is more difficult with a double because of the ribs but still works to keep the muzzle square right at the beginning and eliminate a lot of squaring up work later.

Then it's only a matter of using a fine cut file to remove the file marks.
Use the file to lightly break the sharp outside edges, then wrap fine wet or dry sand cloth around your thumb and lightly break the inside edges of the muzzles.
If you want you can finish with cold blue but it won't last long.

Cut it off close to 19" to give you plenty of room to even things out.

I use a large flat file and a 90 degree square to get the barrels flush.  I use my finger and run some 220 sandpaper on the inside to smooth things out.

Be aware there will probably be a gap in the area between the barrels.  They are normally just filled at the end

I think it is silver solder.

Finished it a couple weeks ago...

Barrels cut.  Bead installed.  Filled rib with JB Steel Stick, filed/sanded.  Hit with cold blue.  Done.

For a one-off project in a steel rib, any of the following can be used with reasonably good results: Bar soap with a touch of water (Ivory or other real soap), bee's wax, WD-40 (though much better for aluminum), just about any light household oil (3-in-1 or similar), non detergent motor oil, even Vasoline will work.  These substitutes won't give as long a tap life that the good cutting fluids do, but they will make tapping easier than dry which reduces the likelihood that you will break the tap off in the hole.  For drilling, just go dry.  

ETA: Late to the party.  But, the information above might help someone else in a similar position.

I used Ballistol