While a shim may work, I don't think it is a proper fix.
Since I have no idea what type gun you have. If it's one with the drum (or vent) screwed through the breech plug (CVA, Traditions, Pedersoli, ect.) this method will not work! You will need a new breech plug.
If you have a patent breech plug the proper way would be to machine the rear of the barrel and the face of the breech plug until it is in the proper location and the breech plug contacts the shoulder in the barrel with 35-40 lbs of torque. This method will move the barrel rearward 50+ thousanths of an inch and may require some work on the under lugs for the pins or keys (wedges) to fit.
If you have a drum (or vent) screwed into the side of your barrel, machine the tang portion of the breech plug where it meets the barrel and also machine the breech plug face untill the breech plug face fits snug against the shoulder in the barrel and the tang lines up with the top flat of the barrel with 35-40 lbs of torque.
It may be eaisier to fit a new tang! This method will move the tang ahead 50+ thousanths of an inch and will require a filler behind the tang, but the barrel will remain in the same location.
Either way assuming your barrel has a 5/8x18 thread you would need to take about 50 thousanths of an inch off each as with 18 threads per inch, 1 rotation of the barrel will move the breech plug 55.5 thousanths of an inch.
As you can see to do it right is not an eaisy job!
SC45-70