If you are looking for that sqaut fat Civil War Mortar profile don't make the bore 1.75 to 2 X longer than the can. The general rule of thumb for these mortars was to have the bore length 2X times the bore diameter. A can is 2.6 inches in diameter, so 2x would be a 5.2 inch bore. A can is 4.835 long. To accomodate a 2x bore, the bore will have to be 9.7 inches long, plus chamber length plus 1x thickness at back of chamber.
For safety purposes you want the entire projectile inside the bore. With the 2X bore you get that.
With the 2X projectile bore you also get protection of a bore longer than projectile, but you loose the profile. I'm not so sure you gain anything, you might even loose a bit given the reduced charges of the mortar with the longer barrel.
Just to be clear when I say bore here, I mean projectile bore and that doesn't include chamber.