It's been a while since I looked at one up close, but doesn't the 'snail' for the TC percussion breech have a groove on the underside for the lock plate? In other words, the top of the lock is flat, and not 'notched' for the snail, and the snail fits over the top of the lockplate?
Anyways, the critical thing will be getting the hammer lined up properly for the nipple. Hopefully, the two are close enough, and there is enough wood around the lock and back end of the barrel to get proper forward or back alignment. Also, if the lock is not inlet deep enough already, you'll have to set the lock deeper to get the notch in the bottom of the snail to align with the top of the lock.
you should have the most available wood to move behind the barrel. This will let the barrel come further back to get front to back alignment of the nipple and hammer. Hopefully the lock and barrel inlets have enough wood left to allow the proper location.
Basically, you will have to watch forward and back locations of barrel AND lock, and in-out location of the lock to line up with the notch, ALL at the same time. All this to make sure the hammer will strike the nipple and cap properly.
So you cant be inletting one part without constantly checking that the mating part(s) are still aligned.
Oh yeah, and then there's the barrel wedge pin location like Eric mentioned. Ouch. sure hope all those 'preset' inlets are close enough to make it work, but there's always ways around obstacles. Some pics would help