IF I were to make a rifle with a drum and nipple,I would first locate the face of the breech plug. I would then check the diameter of the threads on the drum and then,using the face of the plug,locate the place in the flat where it will go. THIS would be done after finding the center of the flat and in this instance,IF it is a 5/8 to 9/16 barrel,the center line would be 5/16ths of an inch from the bottom flat. I would think a 1/4x28 thread would be about all this little barrel could use and I would consult a drill/tap chart and see what is the proper drill for this thread and then drop down one size. After drilling,countersink the hole the depth of one thread and then,after making sure the tap is started straight,put a few drops of tapping fluid or cutting oil on the tap and begin the job. Use a new tap because they cut easier and are less likely to break.
I agree with Dan,a drum and nipple are the least desireable but if a bolster breech isn't available the it's either drum and nipple or make it into a flintlock and having spoken with the man making this rifle,I know the boy isn't interested in a flintlock for a first muzzleloader.
Bob Roller