Count me amoung the completely confused as to what the problem, or percieved problem is here. The lock nail can go anywhere through the breech plug bolster or not at all, doesn't matter. Some are simply notched at the back to allow screw clearence.
Heck, you could grind the thing off all together after installation and the gun would still funtion fine if built correctly....though removal of the plug later may be problematic.
I do suggest that you make that hole in the bolster oversized or even slightly slotted though. It should be a clearence fit, i.e. not tight. If the hole is tight and the barrel sets back at all when the gun if fired the first time the screw can become pinched or even bent some. Hard to get it out that way.
Another thing that I'd like to point out....it appears that you filed a bevel on the tang but didn't on the bolster. I suggest that you taper the bolster the same. It will make barrel removal a little easier.
Good luck and Enjoy, J.D.