One of the things I have done, is taper the metal. This allows a real tight fit. Yes, you can file down the tang.
I like my drum threads to go into the breech plug. I drill into the face of the breech plug. One of the reasons I use a register mark on the breech/barrel intersection, is to be able to reregister the two if I need to remove the drum.
If you taper the tang, it makes it just a tad easier to inlet.
With the taper, you can pencil the part that needs to be inlet then cut away. The pencil mark will be on the narrow part so any oopsies can be compensated for.
The other thing I do, is completely strip my locks to inlet. I also taper those. A quick file making a small entry angle and the lock plate fits well.
If you oopsie with a screw hole, drill it out, then use epoxy and a dowel. I find that using the jaws of a pair of pliers will allow air to escape as the dowel is driven out. Do not force the dowel into the hole. If it is near the buttstock, it can split the parent wood with the pressure of the dowel.
Good luck.