First, there isn't a proper type of shell for old shotguns. Paper, plastic, or brass are all fine. Your shotgun doesn't know what kind of shell you're using or what kind of crimp it has. I shoot every week one of my old SxSs [ I have 9 Remingtons, 3 Parkers, and a Lefever ] and my normal crimp is fold crimp, not a roll crimp. Second, the last thing I'd do is remove any metal near the chamber, like making the chambers longer or making the forcing cones longer. The chambers and forcing cones are where the greatest pressures are produced and you want all the metal you can get. The manufacture designed the barrels with so much steel. Better to leave them that way.
Chamber lengths. Sherman Bell in some of his test where the guns were suppose to be blown up, used 3 1/2" 10ga shells in the old 2 7/8" 10 gauge chambered guns. Pressures went up about 500psi. After reading that I quit measuring the chamber lengths in my old SxSs. I know Remington use 2 9/16" in their 12ga chambers. I normally don't cut a shell down. I also have some 10ga Parkers with 2 5/8 and 2 7/8 chambers. In those I use 3 1/2" shells cut to 2 5/8". The reason is the 2 5/8" chambered Parker has "stepped" chambers. There is no forcing cone, just a .040 step. I believe it was made for brass shells. It was just easier to cut them all 2 5/8 - no sorting between the 5/8 and 7/8 shells. I'll leave 12ga shells with the extra 1/4" and the 3.5" 10ga shells I'll cut to the 2 5/8.
Where plastic shells will burn through when using BP is around the brass base where it meets the plastic or paper. You'll start getting pin holes. If you let it go too long trying to get another load out of them is too many pin holes and the shell will separate with the paper or plastic going down the barrel, hopefully out. That part of the shell staying in the barrel and then you have an obstruction and may blow up your gun.
Now Hawg has me looking down in the shells after firing BP.
I thought it was enough to just check the outside for pin holes. On second thought I'm not gonna. If those fiber wads will go in, that's enough for me.
Pin holes, or little balls of plastic on the inside, either one happens fast enough it ain't gonna matter. Still have to throw them away.