Is the touch hole already drilled? If so I would inlet the lock at the point where you know it has to go, get it firmly in its place, then inlet the barrel (you will have to remove the tang) aiming for the touch hole centered in the pan normally. Once the barrel is in then do the tang and bring the lock the rest of the way in. If the touch hole is not drilled mark where it will go and proceed the same way but you have a bit of wiggle room. Also, if you are going to use a touch hole liner plan that in in regards to where the breech plug will end, usually 1/2 inch from the rear of the barrel plus one half the liner dia. so you don't end up having to drill into the breech.
dave