I just did another check of dimension on your barrel , on the point where it got its smallest diameter just behind the muzzle reinforcement it is approximately 1 5/8" in diameter .
in my opinion it should be 2 1/8" , so its 1/2" too small in diameter there also .
a good rule of thumb is 2 times the bore diameter on the barrels smallest diameter and 3 times the bore in the other end
you can still save the barrel if you have someone to make and install a steel liner in it
49/64 ( 0,765625" ) would be a suitable bore diameter to shoot a 3/4" ball
then you are very close to the 1/40th clearance between ball and bore diameters (called windage)
with a 49/64" bore you could use either fg or cannon grade powder
I know it may sound silly that a liner with just 5/32" extra wall thickness may make that large difference
but it both reduce the bore diameter and add thickness to the walls so the proportions will be very different
please remember that those numbers are just approximate , I live in sweden and got hughe problems to think in both fractional and decimal inch . so please check them carefully before you do anything else .
I think they are correct but can never be sure when I must think in fractions
at the same time I would change the trunnions to shrink fit 3/4" diameter
drill and ream a 3/4" hole all the way through the barrel and shrink fit a slightly over sized bar into the hole.
then drill the bar away that crosses the bore and ream it again for the liner
then you have something that not only looks good but also is safe to shoot
I think that would be the best solution on the problem ,
its a pity to cut such an beautiful barrel into pieces and throw them away .
and not to mention the financial lost
Im sure that you got a friend or neighbor who could help you with this , if not start looking in the sponsor list and ask around for a good price .