I have a Stoeger SxS, and approved form 1, and a miter saw -- but I've no experience in cutting metal.  What sort of blade is best for such a job - carbide high tooth count?

Use a bench vise, a fine toothed (32TPI if possible) bimetal hacksaw blade, and take your time. It's easy to screw up the cut on a power saw, and you're only needing to cut the one shotgun.

Mark where you want the cut by measuring out from the chamber end and placing some masking tape all the way around the barrels, confirm the length again and draw a pencil or pen line on the tape right where you want to cut. Place the barrels in the vise with leather, wood, or some felt or similar material to keep the jaws from marring the barrels, and slowly cut with the hacksaw. No need to rush, do it right. Cleanup with gentle file strokes, and you'll end up with something of which you can be proud!