I would start off with the barrel. Usually, the barrel channel in the wood is either slightly undersize or it is a Very tight fit. It should be sized so that the barrel goes into the stock very easily. Use a block of wood and some sandpaper to keep the surfaces flat.
I would then use a drop of 'super-glue" to glue the tang onto the barrel and lower the barrel/tang into the stock.
You might need to remove any rounded areas left by the router where the rear if the channel meets the flat face at the rear. Use a small, very sharp wood chisel to remove the rounded surfaces.
Although the lock may not fit down into the lock mortise at this time, use the mortise to position the lock and check the alignment of the bolster with the lock plate. You might not have to move the barrel aft to get things to line up but if you do, now's the time to do it.
When the barrel/tang goes down into the stock but the tang stops on the groove that is cut for it, hold the barrel down into the barrel channel and use a very sharp Hobby Knife like an Exacto to cut down along both sides and the rear of the tang. This takes many cuts before the tang will fully seat but when your done, the wood to metal fit will be "line to line" with no visible gaps.
All of this takes time. It is not a "drop in" sort of stock so don't be in a hurry.
Here's a picture of the Pecatonica River stock I used for my T/C Hawken.
