I am real uncomfortable with the flat plate bottom. Assuming only 1000 psi chamber pressure, it will develop over 60,000 pounds force. The weld should be sufficient but the plate will have a tendency to bend which will try to stretch the weld and eventually cause a failure. There was a fatal "cannon" accident reported where a flat breech plate welded to the back of the barrel blew off and hit the deceased. The plate needs to be thick enough to prevent significant bending under chamber pressure.
The long barrel makes this more of a flank howitzer than a mortar. Mortar barrels (at least Civil War era mortars) are usually only about two calibers (bore diameters) in length. The one advantage of using this design in a Stokes configuration is that the back plate wouldn't go very far if it detached (it would still be more exciting than necessary.)
See the drawing below for a M1861-style BB mortar. A real M1861 8 inch is 16" in overall diameter so this one would seem a little thin looking, but the resulting underweight would be nice. Estimated weight 350 lbs.
Just in case it isn't obvious, this design uses a piece of seamless tubing bored 8.65" to a shoulder, and a solid breech plug (inserted from the muzzle) with a matching shoulder containing the powder chamber. They should be press fit together, then welded at the rear (they could also be TIG welded at the front to prevent corrosion from getting in that joint.) The vent should be drilled in a piece of all thread and screwed in to provide a passage through continuous metal to prevent any corrosion or lingering sparks at the joint between the tube and plug.
