Well, I'm not sure what your Missouri rifle is built like, but as you know, there are several different types of Precussion systems.
The type with a "snail" which has the nipple hole as an integral part of the breecn would require replacement of the entire breech.
Rifles with this type of breech are the TC guns, the Lyman Great Plains etc.
Then there is the precussion system which uses a side drum. Rifles that come to mind are some of the older CVAs, Junkers, and most of the Custom Precussion Kentucky/Tennessee rifles.
This side drum just screws into a threaded hole in the side of the barrel and it has little if anything to do with the breech plug.
Speaking of the side drum system, the main requirements of installing one is that the threads are trimmed back so the threaded area does not project into the bore of the gun.
The other requirement is that the nipple is properly aligned with the hammer. This is not always easy to do if the nipple hole is pre-existing like Dixie says their replacement drum is.
When building a gun with this side drum system, the drum is screwed tightly into the barrel. The barrel is then installed into the stock with the lock. The hammer on the lock tells the builder where to locate the hole for the nipple. After drilling and tapping the threads in the nipple hole, if everything was done correctly, the nipple will be aligned with the hammer.
With the pre drilled drum like Dixie sells, the game plan would be to install it and hope the nipple hole is close to where it should be.
If it is just a little below the hammer nose, adding a little more torque to tightening it might align things. If it does, your in business.
If it doesn't, my method would be to remove a little material from the shoulder of the drum, adjacent to the threads. This would allow it to screw in a little further thereby allowing the nipple to rotate up to align with the hammer.
A cruder but not necessarly worse method would be to tighten the drum in the barrel. If the nipple rotates past the hammer nose before the drum is tight, remove the drum and apply LocTite thread locking compound to the threads. Screw the drum in until it bottoms and then back if off until the nipple aligns with the hammer nose. Let it sit there while the thread locking compound cures.
If everything is "right", the drum will be supported by the lock when the barrel is installed. Because of this, the threads really don't have to absorb the impact of the hammer hitting the nipple so the only thing they have to do is to keep the drum from blowing out the side of the barrel when the gun is fired.
Many of the less expensive or poorly made guns locate the drum up above the lockplate so the drum is not supported. In these guns, the threads have to take all of the shear force of the hammer hitting the nipple. This can cause the threads to fail, and I wouldn't be suprised to find that the gun which is the subject of this post failed for exactly this reason.
It's not PC by any means, but if your gun has a side drum which is not supported by the lockplate, it is a good idea to apply some grease to the outside of it. Then mix up some strong Epoxy, the kind with steel fillers in it and apply a little to the lock plate where the drum will fit. Reinstall the barrel letting the drum settle in this Epoxy bed until it hardens. Remove the barrel and file/sand off the extra Epoxy from the sides of the lockplate. This will help absorb the impact of the hammer when it hits the nipple.
(Your Results may vary)
Zonie